A batch of 4 products is known to contain 2 that are defective. The products are to be tested, one at a time, until the defective ones are identified. Denote by X the number of tests made until the first defective is identified and by Y the number of additional tests until the second defective is identified.
(a) Find the joint probability mass function of X and Y.
(b) Find P(X ≤ Y).
(b) Find E[X + Y].