Each day Chen buys pasta or a sandwich. Where Pasta costs £3.20 and Sandwich costs £2.75. For 45 days, the relative frequency of Chen buying a sandwich was one fifth. Altogether, how much did he pay?

Firstly work out how many days Chen bought a sandwich, which was one fifth of 45 days. So 45/5 = 9. We then multiply 9 by the cost of a sandwich, 92.75 = 24.75. The remaining days Chen must have bought Pasta, therefore Chen had Pasta for the other (45-9) 36 days. We multiply that by 3.20, so 3.2036 = 115.20. Altogether he paid £139.95, which is attained from adding 24.75 and 115.20.