You have to watch it with these multiple-roll packs. Tesco is offering a 9 pack of Andrex for £4.50 but the 4 packs are on special offer at 2 for £3.50!
Your maths is dodgy, anya but so is theirs. You have the sum the wrong way round. If you divide the £4.50 by 1440, you get 3.125 pence per sheet, the right answer. They have done rounded it up and put the decimal point in the wrong place
You divided 1440 by 4.5 to get 320 - and then put in the decimal point in the wrong place.