In a set of magicians cards, there are 15 red cards, and 60% more green cards. Yellow cards are as many, as the sum of red and green cards. How many cards of all mentioned colors are there?
Give me a clear explanation. 
There are 60/100 * 15 = 9 more green cards than red cards.
Which means there are 15 + 9 = 24 green cards.
Yellow cards make up to 15 + 24 = 39 cards.
So in total, there are 39 + 39 = 78 cards of different colors.