Johns collection contains us, indian and british stamos. if the stamps ratios of us to indian stamps is 5 to 2, and the ratio of indian stamps to british stamps is 5 to 1, what is the ratio of us to british stamps

Figure out what is the commonality for each ratio.
There are India stamps in each ratio, so India is the commonality.

Now you simply take the two ratios and multiply them until India has the same amount.

So, Multiply the first by 5 to get a ratio of 25 US to 10 Indian
Multiply the second by 2 to get a ratio of 10 Indian to 2 British

Now that you have "10 Indian" in both ratios, they match up
You have 25 US : 10 Indian : 2 British.

Since the answer is only asking for US and British, you only need 25 US : 2 British

It’s either a or c as the answer.