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.