There are 55 cakes. 20 boys and 19 girls each take a cake. How many cakes are left?
We can sketch a bar model to help us think about this problem. We know that there are 55 cakes to begin with. We know that 20 boys each take a cake and 19 girls each take a cake. We need to work out how many cakes are left. If we add the 20 cakes the boys ate and the 19 cakes the girls ate, that will tell us how many cakes have already been eaten.
19 is one less than 20. If we add one to 19, that will make our calculation easier. We need to remember to take this one away from the answer. 20 and 20 or two tens plus two tens equals four tens which is 40. And remember we have to take away the one that we added to 19, which gives us a total amount of cakes eaten of 39. The children have eaten 39 cakes and there are 55 cakes.
To work out how many cakes are left, we need to find the difference between 39 and 55. If we add one to 39, we will have 40. To get from 40 to 50, we have to add 10. And to get from 50 to 55, we need to add five. Altogether, we’ve added one ten and five. One and 10 is 11 and five more is 16. There are 16 cakes left.