20% of the people in a school hall are teachers and the rest are pupils. 75% of the pupils are boys and there are 72 more boys than girls. How many boys must leave the hall so that 20% of the people in the hall are boys?

Emily earns a commission on 12% for the first 5 encyclopaedias she sells. For any additional sales, she will get a 15% commission.

Each encyclopaedia is sold at $140. If she receives a total commission of $462, how many encyclopaedias does she sell altogether?

A slice of cheesecake cost $4. A bun cost 25% as much as a slice of cheesecake. Alice paid $154 for some slices of cheesecake and buns. 40% of what she bought were buns. How many slices of cheesecake did she buy?