Knowledge is Silver?

Description

May 12

There are exceptional scholars in Calpheon. They are especially known for their astronomy and mathematics.

Today I made a bet with one of the scholars at the library. I hurt his pride on purpose because he was from a rich family.

I'm sorry, but my only goal right now is to make money.

He was a mathematician and he boasted that he would pay me a load of silver if I was able to answer his question. Ugh, just the thought of him make me rage.

I receives only half the amount he offered because I wasn't able to answer him within the time limit. It was fortunate that I got the ruby in advance.

I was so infuriated at that time, I still remember his question.

Hmm... but what was the answer?