Subtraction: bridge through 10
for Kindergarten

Kindergarten : Subtraction :
Subtract a single digit number from a double digit number

In this topic we are going to look at taking away (subtracting) a single digit number like 4 or 7 from a double digit number like 12 or 15 (this is sometimes called 'bridge through 10' because you are counting down from double digits to single digits through the number 10). This is a little bit harder than subtracting single digit numbers but not much harder. So, how can we work out the answer to the following sum :

12 - 7

We know we can subtract numbers by counting down from the first (bigger) number by the same number of steps as the second (smaller) number. So to subtact 6 - 3, we start with 6 and count down three places to 5 then 4 then 3, and 3 is the answer.

When we are subtracting from a double digit number, we do the same thing. So, for 12 - 7, we are going to start with 12 and count down seven places - so, 11, 10, 9, 8, 7, 6, 5. So we can say :

12 - 7 = 5

If you find counting down hard, a different way to work out the sum is to start with the smaller number and count up to the bigger number, and the number of steps you take will be the answer. So if we have the sum 9 - 5, we can count up from 5: to 6, then 7, then 8, then 9. So that was 4 steps and the answer is 4. For 12 - 7, we would start at 7 then :

1. up 1 to 8
2. up 1 to 9
3. up 1 to 10
4. up 1 to 11
5. up 1 to 12

The number of steps from 7 up to 12, which is the answer, is 5

Another question from the topic is :

17 - 8

So let's use our counting up method. We start at 8, plus 1 to 9, plus 1 to 10, plus 1 to 11, plus 1 to 12, plus 1 to 13, plus 1 to 14, plus 1 to 15, plus 1 to 16, plus 1 to 17. How many steps was that? It was 9 steps so we can say :

17 - 8 = 9