Urn problem (draw two balls of different colours with replacement) e.g. 3 white balls and 7 black balls.
Prove that the probability to draw two different coloured balls is 0.42 
I get the intuition, but can't formally explain the probability.

Or alternately, you can think of picking a white ball first and then a black ball. Or, a black ball first and then a white ball. that's 3/10 * 7/10 + 7/10 * 3/10 = 21/100 + 21/100 = 42/100
