b) True.

As A and B have more than 10 coins for 50¢, the balance must be less than C no matter how many coins they have in total. Hope this clarifies your doubts.

All three children had the same number of coins to start with. Let us call this number x.

So, the value of the coins that each child has is as follows (in terms of x):

Amy: 18 ($0.50) + $(x - 18) = $ (x - 9)
Brendon: 12 ($0.50) + $(x - 12) = $ (x - 6).
Chandra: $x

However, the thing is that the price of each file is not given. All that we know is that each child used 10 of their coins to buy the files.

(a) It is not possible to tell whether Amy spent more than Brendon, since the price of each file is not given, and we are not told which coins they used.

(b) Although Chandran has the most money to start with, it is not possible to tell how much money each of them will have in the end, since we do not know which coins Amy and Brendon used.

Amy and Brendon have more than 10 50¢ coins, (18 and 12 respectively), if they use all 50¢ coins, they still have balance of 8 or 2 50¢ coins. So, in the end, their total value should be less than Chandra?