Here is a maths worksheet on using mental arithmetic to subtract from 100. This is probably the type of calculation we do more than any other, especially when working out change from £1.00. There are several processes or ways to do these.
Let’s look at an example: 100 – 47.
One way is to add on 3 to make 50 then 50 to make 100: add the 50 and the 3 to make 53.
Another way is to add 50 to make 97 then count on 3 to make 100.
Watch out for the very common error that children make which is adding 3 to the 7 and adding 60 to the 40 and coming up with the answer 64!