Addition and subtraction to 100 — for parents

Adding and subtracting in the 0–100 range is the biggest math skill of 2nd grade. Without it, the 3rd-grader can't move on to multiplication, word problems, money, or measurement. Your child arrives from 1st grade able to add and subtract within 20 — now the range expands fivefold and two new mechanisms appear: regrouping (carrying) and borrowing.

Why this is the make-or-break year

Before 2nd grade, your child added mostly by memory — they knew 7+5=12 as a learnt pair. In 2nd grade memory runs out (there are too many combinations). The child has to internalise a procedure: ones first, then tens, with the regrouped ten properly moved across.

The very same procedure is what the 3rd-grader uses for numbers up to 1000 (with hundreds added), what the 4th-grader uses for numbers up to 10,000, and frankly what any adult uses for written arithmetic. We're teaching the 2nd-grader to work with place values — a lifelong investment.

The most common mistakes

1. The carried ten is added to the ones but forgotten in the tens.

For example 27 + 18 — the child writes the ones correctly (5) but does the tens as 2 + 1 = 3. Result 35 instead of 45. Always ten too small.

Fix: have the child write a tiny 1 above the tens during regrouping, and physically run a finger over it as they compute the tens. What's written is harder to forget. 2. After borrowing, forgetting that the top tens digit is now one less.

On 63 − 28 the child borrows a ten correctly, computes the ones correctly (13 − 8 = 5), then for the tens does 6 − 2 = 4 instead of 5 − 2 = 3. Result 45 instead of 35.

Fix: have the child cross out the original tens digit and write the smaller one above it. A visible reminder it's now one less. 3. Misalignment in the column method.

The child writes a ones digit under a tens digit or vice versa. The result is absurd.

Fix: squared notebook. One digit per square. Or draw two vertical columns labelled "T" and "O" at the top. 4. In subtraction, mixing up which subtracts which.

On 52 − 27 the child might do the ones as 7 − 2 = 5 (because 7 is bigger), which is wrong.

Fix: repeat the phrase "top minus bottom", always. If the top isn't enough, we borrow.

Activities for home practice

A daily ten-minute session beats a long Saturday morning. Five minutes a day is better than an hour a week.

Coins and tens. Set out a pile of coins (mixed 10 and 1 of your local currency). Child first counts the value, then either adds more coins to reach a target (e.g. 80) or removes a smaller pile.
Cards in columns. Cut twelve cards numbered 10–99. Flip two at random. Child writes them one above the other and adds. Best of three pairs wins.
Real-life prompts. "We have 23 apples in the basket and we'll buy 18 more — how many will we have?" "There are 45 eggs in the fridge, we cook twelve — how many are left?"
100 pyramid. Build a triangle: one number at the top (up to 100), two numbers below that sum to it. Harder, but fun.
Backwards — from result to problem. "I've got the result 53. Find two numbers that add up to it, and both must be at least 20." Open-ended tasks build flexibility.

When to seek extra help

Most children handle this in 4–8 weeks of regular practice in 2nd grade. We'd recommend talking to the teacher or a specialist if:

After several months your child still can't separate tens from ones in 47 (says, e.g., "4 ones and 7 tens").
They get stuck on the tens crossover — still doing it shakily after two months of practice.
Adding within 20 still isn't automatic (e.g. 7+5 still requires finger-counting). That signals a weaker 1st-grade foundation.
They avoid maths and say "I can't" even on easy problems.

These signs can point to dyscalculia or weaker working-memory processes. Early detection and gentle support make a big difference.

Summary for the parent

Adding and subtracting within 100 is the key skill of 2nd grade — nothing else moves on without it.
The most important trick (and most common mistake) is the regrouping step — adding the "extra ten" to the tens column.
Short daily practice with real objects (coins, cards) beats long homework sessions.
If difficulties persist, talk to the teacher sooner rather than later.