Question

An experiment consists of choosing a colored urn with equally likely probability and then drawing a ball from that urn. in the brown urn, there are 24 brown balls and 22 white balls. in the yellow urn, there are 15 yellow balls and 5 white balls. in the white urn, there are 21 white balls and 13 blue balls. what is the probability of choosing the yellow urn and a white ball?

Answer 1

So there are 2 parts, first you have to choose the yellow urn. 
There are 3 different urns, so the chance to pick the yellow is 1/3

In the yellow urn there are 15+5=20 balls total out of which there are 5 whites, so the chance is 5/20=1/4

These are indipendent events thus you have to multiply the 2 probabilities to get the answer: 
1/3*1/4=1/12