🧠 Mental Math for Adding and Subtracting

What you’ll learn

1. Add Ones in Your HeadMentally add a one-digit number to a two-digit number by bridging the next ten, like 36 + 7 = 36 + 4 + 3 = 43.To add in your head, bridge over the next ten. For 36 + 7, notice 36 needs 4 to reach 40, so split 7 into 4 and 3: jump to 40, then add 3 for 43. Picturing the two jumps on a number line makes the trick stick, and it works for any sum like 27 + 5 = 32.
2. Subtract Ones in Your HeadMentally subtract ones from a two-digit number by resting at the ten, like 52 − 4 = 52 − 2 − 2 = 48.Mental subtraction hops backwards with a rest stop at the ten. For 52 − 4, hop back 2 to reach 50, then 2 more to land on 48. Splitting the number you subtract, like 6 into 5 and 1 for 45 − 6 = 39, keeps every hop easy — and adding back (48 + 4 = 52) checks your work.
3. Money ProblemsSolve two-step money problems like 35p − 12p + 5p = 28p by working one step at a time.Money problems are number problems in disguise, and two-step stories need two calculations. Zoe starts with 35p, spends 12p (35 − 12 = 23), then finds 5p (23 + 5 = 28). The secret is order: read the whole story, do step one, keep the middle answer, then finish with step two.
4. Pick Your MethodChoose sensibly between mental math and written methods, and check that answers make sense.Different problems deserve different tools: quick facts like 8 + 2 belong in your head, while big sums like 36 + 47 are safer on paper. After solving, do a sense check — 52 − 4 must be a little smaller than 52, so 48 fits. Choosing the method and checking the answer is what real mathematicians do.

Questions this course answers

Grounded in trusted sources

• Khan Academy Kids
• PBS Kids — early math
• NAEYC — early math learning
• Britannica Kids

Every Wunder lesson is built from real, reputable sources — never invented.

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