Adding and subtracting in your head

You add and subtract in your head every day: the change from a shop, the minutes until the bus, the points in a card game. You do not need paper for numbers under 100. You only need one simple idea: split each number into its tens and its ones, work with each part, and put them back together. This page shows the plain method first, then a few quick tricks, and also how to double and halve a number fast.

Tens and ones

Every two-digit number is made of two parts. A digit is one of the signs 0 to 9. In 47, the digit 4 counts tens and the digit 7 counts ones. So 47 is 4 tens and 7 ones, or 40 + 7.

Tens are easy to add, because you only add the tens digits and put a zero after them: 20 + 40 = 60, just like 2 + 4 = 6. That is why we split.

Adding two-digit numbers

1. Add the tens digits, as tens.
2. Add the ones digits.
3. Add the two answers together.

Subtracting two-digit numbers

For subtraction, keep the first number whole and take the second number away in two small jumps: first its tens, then its ones.

When the second jump crosses a ten, it helps to do it in two parts. To take 7 from 52, first take 2 to reach 50, then 5 more to reach 45.

Doubles

To double a number means to add it to itself, or multiply it by 2. Double 7 is 14. To double a bigger number, double the tens and the ones on their own, then add.

Double 48 works the same way: double 40 is 80, double 8 is 16, and 80 + 16 = 96. A three-digit number can be split into hundreds as well: double 125 is double 100 plus double 25, so 200 + 50 = 250.

Halves

To halve a number means to split it into two equal parts, or divide it by 2. Halving undoes doubling: half of 74 is 37 because double 37 is 74. Split the number into parts that are easy to halve.