Jo's collection contains US, Indian and British stamps. If the ratio of US to Indian stamps is 5:2 and the ratio of Indian to British stamps is 5:1, what is the ratio of US to British stamps?

The correct option is C

25:2

Indian stamps are common to both ratios. Multiply both ratios by factors such that the Indian stamps are represented by the same number.

US : Indian = 5 : 2, and Indian : British = 5 : 1. Multiply the first by 5, and the second by 2.

Now US : Indian = 25 : 10, and Indian : British = 10 : 2

Hence the two ratios can be combined and US : British = 25 : 2.