Let X1 and X2 have the joint pmf p(x1, x2) = x1x2/36, x1 = 1, 2,3 and x2 = 1, 2, 3, zero elsewhere. Find first the joint pmf of Y1 = X1X2 and Y2 = X2, and then find the marginal pmf of Y1

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?