{"id":131,"date":"2026-09-22T23:41:56","date_gmt":"2026-09-22T23:41:56","guid":{"rendered":"https:\/\/quantumleapstar.com\/?p=131"},"modified":"2026-09-23T18:52:51","modified_gmt":"2026-09-23T18:52:51","slug":"year-8-maths-adding-and-subtracting-decimals","status":"publish","type":"post","link":"https:\/\/quantumleapstar.com\/?p=131","title":{"rendered":"Year 8 Maths: Addition and Subtraction of Whole Numbers and Decimals"},"content":{"rendered":"<p><strong>Year group:<\/strong> Year 8 (Key Stage 3) &nbsp; <strong>Topic:<\/strong> Addition and subtraction of whole numbers and decimals &nbsp; <strong>Difficulty:<\/strong> Foundation to intermediate<\/p>\n<p>You can add and subtract without a calculator by lining up digits with the same place value. Work from right to left. The examples below show when to carry or exchange, and why decimal points must line up.<\/p>\n<h2 style=\"background:#49377b;color:#fff;padding:10px 14px\">Watch the video lesson<\/h2>\n<p>This narrated Year 8 lesson is 3 minutes 20 seconds long. Watch the worked examples, then pause at the practice slide and try the four questions yourself.<\/p>\n<figure class=\"wp-block-video\"><video controls preload=\"metadata\" width=\"1280\" height=\"720\" src=\"https:\/\/quantumleapstar.com\/wp-content\/uploads\/2026\/09\/Year_8_Addition_and_Subtraction_Lesson.mp4\">Your browser does not support embedded video. <a href=\"https:\/\/quantumleapstar.com\/wp-content\/uploads\/2026\/09\/Year_8_Addition_and_Subtraction_Lesson.mp4\">Open the video<\/a>.<\/video><figcaption>Year 8: addition and subtraction of whole numbers and decimals (3:20).<\/figcaption><\/figure>\n<figure class=\"wp-block-image size-full\"><img decoding=\"async\" src=\"https:\/\/quantumleapstar.com\/wp-content\/uploads\/2026\/09\/year8_column_methods.png\" alt=\"Four column examples: 342 plus 78 equals 420; 372 minus 234 equals 138; 47.32 plus 1.65 equals 48.97; 7.200 minus 3.864 equals 3.336.\" style=\"max-width:100%;height:auto\" \/><figcaption>Original Year 8 column-method guide. Follow each method below.<\/figcaption><\/figure>\n<h2 style=\"background:#49377b;color:#fff;padding:10px 14px\">Adding whole numbers<\/h2>\n<ol>\n<li>Write the numbers in columns: ones under ones, tens under tens.<\/li>\n<li>Start at the right. Add the digits in each column.<\/li>\n<li>If a column totals 10 or more, write its ones digit and carry the extra ten to the next column.<\/li>\n<\/ol>\n<p><strong>Worked example: 342 + 78.<\/strong> Ones: 2 + 8 = 10, so write 0 and carry 1 ten. Tens: 4 + 7 + 1 = 12 tens, so write 2 and carry 1 hundred. Hundreds: 3 + 1 = 4. Therefore <strong>342 + 78 = 420<\/strong>.<\/p>\n<h2 style=\"background:#92b657;color:#15240e;padding:10px 14px\">Subtracting whole numbers<\/h2>\n<ol>\n<li>Line up digits in the same place-value columns.<\/li>\n<li>Work from right to left.<\/li>\n<li>If the top digit is too small, exchange 1 from the column to its left for 10 in the current column.<\/li>\n<\/ol>\n<p><strong>Worked example: 372 \u2212 234.<\/strong> Ones: 2 is smaller than 4, so exchange 1 ten. The ones become 12 and the tens become 6. Then 12 \u2212 4 = 8; 6 \u2212 3 = 3; and 3 \u2212 2 = 1. Thus <strong>372 \u2212 234 = 138<\/strong>. Check: 138 + 234 = 372.<\/p>\n<h2 style=\"background:#49377b;color:#fff;padding:10px 14px\">Adding and subtracting decimals<\/h2>\n<p><strong>The method is the same:<\/strong> line up the <strong>decimal points<\/strong>, so tenths are under tenths and hundredths under hundredths. You can write extra zeros at the end: 7.2 = 7.200.<\/p>\n<p><strong>Worked example: 47.32 + 1.65.<\/strong> Hundredths: 2 + 5 = 7. Tenths: 3 + 6 = 9. Ones: 7 + 1 = 8. Tens: 4 remains. Therefore <strong>47.32 + 1.65 = 48.97<\/strong>.<\/p>\n<p><strong>Worked example: 7.2 \u2212 3.864.<\/strong> Write 7.2 as 7.200. Exchange across the zeros: 10 thousandths \u2212 4 = 6; 9 hundredths \u2212 6 = 3; 11 tenths \u2212 8 = 3; 6 ones \u2212 3 = 3. Thus <strong>7.200 \u2212 3.864 = 3.336<\/strong>. Check: 3.336 + 3.864 = 7.200.<\/p>\n<h2 style=\"background:#92b657;color:#15240e;padding:10px 14px\">Before you finish<\/h2>\n<ul>\n<li>Estimate: 47.32 + 1.65 is about 47 + 2 = 49, close to 48.97.<\/li>\n<li>For subtraction, add the answer to the smaller number to check.<\/li>\n<li>For money and measurements, include the correct unit in your answer.<\/li>\n<\/ul>\n<h2>Practice questions<\/h2>\n<p>Try these <strong>15 original questions<\/strong> without a calculator. Write out columns for questions 1\u201310. Worked answers are in the separate section below.<\/p>\n<ol>\n<li>342 + 78<\/li>\n<li>568 + 247<\/li>\n<li>903 + 89<\/li>\n<li>372 \u2212 234<\/li>\n<li>604 \u2212 278<\/li>\n<li>1000 \u2212 456<\/li>\n<li>47.32 + 1.65<\/li>\n<li>3.47 + 2.8<\/li>\n<li>0.086 + 1.35<\/li>\n<li>7.2 \u2212 3.864<\/li>\n<li>8.3 \u2212 2.57<\/li>\n<li>5.004 \u2212 0.86<\/li>\n<li>A book costs \u00a37.85 and a pen costs \u00a31.65. What is their total cost?<\/li>\n<li>A ribbon is 12.5 m long. You cut off 3.75 m. How much remains?<\/li>\n<li>Fill the missing digit: 6.\u25a18 + 1.45 = 8.13.<\/li>\n<\/ol>\n<hr \/>\n<h2>Worked answers<\/h2>\n<p>Check your method after attempting the questions.<\/p>\n<ol>\n<li>342 + 78: 2 + 8 = 10 (carry 1); 4 + 7 + 1 = 12 (carry 1); 3 + 1 = 4. <strong>420<\/strong>.<\/li>\n<li>568 + 247: 8 + 7 = 15 (carry 1); 6 + 4 + 1 = 11 (carry 1); 5 + 2 + 1 = 8. <strong>815<\/strong>.<\/li>\n<li>903 + 89: 3 + 9 = 12 (carry 1); 0 + 8 + 1 = 9; hundreds 9. <strong>992<\/strong>.<\/li>\n<li>372 \u2212 234: exchange one ten; 12 \u2212 4 = 8, 6 \u2212 3 = 3, 3 \u2212 2 = 1. <strong>138<\/strong>.<\/li>\n<li>604 \u2212 278: exchange one hundred into tens, then one ten into ones; 14 \u2212 8 = 6, 9 \u2212 7 = 2, 5 \u2212 2 = 3. <strong>326<\/strong>.<\/li>\n<li>1000 \u2212 456: exchange across the zero columns; 10 \u2212 6 = 4, 9 \u2212 5 = 4, 9 \u2212 4 = 5. <strong>544<\/strong>.<\/li>\n<li>47.32 + 1.65: decimal points aligned; 2 + 5 = 7 hundredths, 3 + 6 = 9 tenths, 47 + 1 = 48. <strong>48.97<\/strong>.<\/li>\n<li>3.47 + 2.80 = <strong>6.27<\/strong>.<\/li>\n<li>0.086 + 1.350 = <strong>1.436<\/strong>.<\/li>\n<li>7.200 \u2212 3.864 = <strong>3.336<\/strong>. Check: 3.336 + 3.864 = 7.200.<\/li>\n<li>8.30 \u2212 2.57 = <strong>5.73<\/strong>. Check: 5.73 + 2.57 = 8.30.<\/li>\n<li>5.004 \u2212 0.860 = <strong>4.144<\/strong>.<\/li>\n<li>\u00a37.85 + \u00a31.65: 85p + 65p = 150p = \u00a31.50; \u00a37 + \u00a31 + \u00a31.50 = <strong>\u00a39.50<\/strong>.<\/li>\n<li>12.50 \u2212 3.75 = <strong>8.75 m<\/strong>. Check: 8.75 + 3.75 = 12.50.<\/li>\n<li>Hundredths: 8 + 5 = 13, so carry 1 tenth. Tenths: \u25a1 + 4 + 1 = 11, so \u25a1 = <strong>6<\/strong>. Check: 6.68 + 1.45 = 8.13.<\/li>\n<\/ol>\n<p><em>Original explanations, illustration, practice questions, and answers for Year 8.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Year group: Year 8 (Key Stage 3) &nbsp; Topic: Addition and subtraction of whole numbers and decimals &nbsp; Difficulty: Foundation to intermediate You can add and subtract without a calculator<\/p>\n<p><a href=\"https:\/\/quantumleapstar.com\/?p=131\" class=\"more-link\">Continue reading<span class=\"screen-reader-text\">Year 8 Maths: Addition and Subtraction of Whole Numbers and Decimals<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[],"class_list":["post-131","post","type-post","status-publish","format-standard","hentry","category-mathematics-year-8"],"_links":{"self":[{"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/posts\/131","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=131"}],"version-history":[{"count":3,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/posts\/131\/revisions"}],"predecessor-version":[{"id":136,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=\/wp\/v2\/posts\/131\/revisions\/136"}],"wp:attachment":[{"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=131"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=131"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/quantumleapstar.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=131"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}