How Do You Add 20 Percent To 12590?

First you find out 20% of 12590 and then you add the amount to this number. You can do it in one equation like:

20 (12590) + 12590
100

= 0.2(12590) + 12590
= 2518 + 12590
= 15108

So, your answer would be 15108.

Usually, these types of problems are set with costs and price. For example, this question might have been:
Find the price of a good A if the seller sells it for 20% more its cost to him.
You would have then used the same methodology as above to solve the question.