Math Brain Teaser! Can't solve, please help!

Question

The sum of face values of two coins is twice the difference of the two. Find them.

My math professor gave us a brain teaser, but I am extremely confused on how to solve it. I am aware that it might be a April Fool's joke, but he still wants a solution.. I know that one coin must be three times the other.

Answer 1

A ratio of three is a rarity in coinage systems. They tend to use $1,2,5$ and things like that. I thought to check the old British coinage, because I knew it had funny values. Sure enough

In the years just prior to decimalisation, the circulating British coins were the half crown (2/6), two shillings or florin (2/-), shilling (1/-), sixpence (6d), threepence (3d), penny (1d) and halfpenny (½d).

So my answer would be a threepence and a penny.