An urn contains 21 balls identical in every aspect except color. There are 7 green, 13 red balls and 1 white ball in the urn.
a. If you draw a ball, record the color, replace it then draw a second, what is the probability of choosing two white balls?
b. If you draw a ball, record the color then draw a second without replacing the first ball, what is the probability of choosing two green balls?