For part a, there is no math formula here but we need to use logic here. Now, 50 cent coins have more value than 10 cent coins so with this, straight away, I know that Cindy definitely has the most money as all her coins are 50 cent. For Beth, she has the most number of 10 cent coins so logically, she has the least money.

Now for part b, take note that for every 10 cent coin more than Beth has, Beth has 1 less 50 cent coin so actually, she loses 40 cent of value. So the difference is 40 cent times 6, which is the difference between the number of 10 cent coins A and B have.

For part C, the qn says Beth uses up all the 50 cent coins so she have only 15 10 cent coins left which is 1.50 and since she has $10 less than Cindy, this means that Cindy has $11.50. To find the number of 50-cent coins, 11 times 2 then plus +1 = 23 since $1 makes up of 2 50 cent coins or u can press calculator to get 23 too.