Two shops have deals for purchasing pens: "3 for £2" and "5 for £3" . Mr. Papadopoulos wants to buy 30 pens for his class in school, which deal should he use if he wants to spend the least amount of money possible, and how much will he spend?

3 for £2,

he needs 30 pens so he needs to buy the deal 10 (30/3) times, meaning he needs to spend 10 * £2 = £20

5 for £3,

he needs 30 pens so he needs the deal 6 (30/5) times, meaning he needs to spend 6 * £3 = £18

£20 > £18 so Mr. Papadopoulos should go for the second deal, as it is cheaper by £2.