Amy buys an ice cream for 90p. There are two parts of the problem. a) Tick three coins to show how Amy can make 90p. And then b) Tick four coins to show another way to make 90p.
We’re told that Amy buys an ice cream for 90 pence. And the problem is really asking us to show how she did it. In the first part of the problem, part a, we’re ask to tick three coins. So we need to make 90 pence using three coins. The coins we have to choose from are a 50-pence coin, two 20p coins, two 10p coins, and a five-pence coin. We don’t have a lot of coins that have a large value. So it looks like we might need to use the 50-pence coin.
Here is a sketch of our 50p coin. But what do we add to 50 to make 90. We know that five and four make nine. So five 10s plus four 10s must make nine 10s. In other words, 50p plus 40p equals 90p. We need to make 90p using three coins. And we’ve already decided to use a 50-pence coin. How can we make the remaining 40 pence using two coins? 20 pence plus 20 pence equals 40 pence. 50 plus 20 equals 70 plus another 20 equals 90. We’ve made 90 pence using three coins. So let’s tick the coins that we’ve used, 50 and two 20s.
Now, let’s think about the second part of the problem. We’re asked now to tick four coins to show another way to make 90p. One of our coins is still gonna need to be 50 pence because all the other coins are too small. So that’s one out of our four coins. But how can we now make 40 pence using the remaining three coins? We could use one 20-pence coin just like before. But we could then swap the other 20-pence coin for two 10p coins. 50 plus 20 equals 70. If we then add 10 pence, that takes us to 80, and then another 10 pence and we get 90p.
So the four coins that we can tick to show another way to make 90p are the 50p coin, a 20p coin — and it doesn’t matter which of the two 20s we’re going to tick. We’ll tick the first one. It doesn’t matter if we tick the second one instead. And then two 10p coins. Three coins that show how Amy can make 90p are 50 pence, 20 pence, and the other 20 pence. And the four coins that we need to use to show another way to make 90p, are 50 pence, one of the 20-pence coins, and then two 10-pence coins.