Impossible

Question: 1.

Apples are packed in boxes of 8 and 15. What is the biggest number of apples that cannot be packed using the above two boxes?

Question: 2.

A country only has 5p and 7p coins. Make a list of prices that you could give exact money for. What is the highest prices that you couldn't give exact money for?

Answer:

"Given integers 'a' and 'b' the biggest number that can't be expressed in the form ia + jb is
ab - a - b. Using this the answers easily turn out to be 97 and 23 for questions 1 and 2 respectively.