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Mental Addition Strategies

Maths · Y3 Add & Subtract
Year 3
Lesson Guide
Explanation
Ten is the friendliest number there is — so when you add, visit it on the way! For 8 + 5, split the 5 into 2 and 3. First jump: 8 + 2 = 10. Second jump: 10 + 3 = 13. Two easy jumps instead of one tricky one. This is called bridging to ten.
Your Turn
Bridge to ten: 7 + 6 and 9 + 8. Check: 7 + 3 = 10, 10 + 3 = 13; and 9 + 1 = 10, 10 + 7 = 17.
Watch Out
Split the second number so the FIRST jump lands exactly on 10. Ask: how far is my number from 10?
Explanation
The split strategy pulls both numbers apart into tens and ones. 34 + 25: the tens are 30 + 20 = 50, the ones are 4 + 5 = 9. Join them back: 50 + 9 = 59. It works because place value lets you add each kind of block separately.
Your Turn
Split and add: 42 + 36, then 53 + 24. Check: 40+30=70, 2+6=8 → 78; and 50+20=70, 3+4=7 → 77.
Watch Out
If the ones add to more than 9 (like 4 + 8 = 12), remember that 12 is a ten and 2 — add the extra ten to your tens total.
Explanation
The jump strategy keeps the first number whole and jumps along the number line. 46 + 23: one big jump of 20 lands on 66, then a small jump of 3 lands on 69. Jumping tens is easy — only the tens digit changes.
Your Turn
Jump it: 35 + 42, then 57 + 26. Check: 35 + 40 = 75, + 2 = 77; and 57 + 20 = 77, + 6 = 83.
Watch Out
Do the BIG jump first (the tens). Small-jump-first works too, but big-first keeps you from losing track.
Explanation
Numbers ending in 8 or 9 are so close to a friendly ten — let them borrow! 39 + 26: pretend the 39 is 40. Now 40 + 26 = 66, easy. But you added 1 too many, so give it back: 66 − 1 = 65. Round, add, fix.
Your Turn
Try it: 48 + 25 and 19 + 34. Check: 50 + 25 = 75, − 2 = 73; and 20 + 34 = 54, − 1 = 53.
Watch Out
The fix-up step is easy to forget! If you rounded UP by 1, you must take 1 AWAY at the end.
Explanation
You now have four tools: bridge to ten for small numbers, split for two 2-digit numbers, jump to keep one number whole, and friendly numbers when something ends in 8 or 9. Good mathematicians look at the question FIRST, then pick the easiest tool. 29 + 45? That 29 is begging to be a friendly 30!
Your Turn
Which tool would you pick for 9 + 7, 52 + 31, and 58 + 27? Then solve them. Check: 16, 83, 85 — any tool that got you there is a good tool.
Watch Out
Speed comes from choosing well, not rushing. Take one second to spot the easiest path before you start.
Step 1: Introduction
Step 1 of 5
8 + 5 — bridge to ten 81013 +2 +3 Jump to 10 first, then add the rest.8 + 5 → 8 + 2 = 10, then 10 + 3 = 13
34 + 25 — split it up 34 + 25 30 + 20 = 50 4 + 5 = 9 50 + 9 = 59 Split both numbers into tens and ones.Add the tens, add the ones, then join them back.
46 + 23 — big jump, small jump 466669 +20 +3 Keep the first number whole. Jump the tens, then the ones.46 + 20 = 66, then 66 + 3 = 69
39 + 26 — round it, then fix it 39 is nearly 40 40 + 26 = 66 borrowed 1, give it back: 66 − 1 = 65 Round to a friendly ten, add, then undo the change.Added 1 too many? Take 1 back at the end.
Your mental toolbox 🧰 Bridge to ten small numbers: 8 + 5 Split both 2-digit: 34 + 25 Jump keep one whole: 46 + 23 Friendly numbers ends in 8 or 9: 39 + 26 There is no single right strategy — pick the one that makes THIS question easy. Now try the quiz!
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