Explanation:

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

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

Hence, option 4 is the correct answer.