Urn I and urn II each have two red chips and two white chips. Two chips are drawn simultaneously from each urn. Let X1 be he number of red chips in the first sample and X2 be the number of red chips in the second sample. Find the pdf of X1 + X2.
solution: 0: 1/36, 1: 2/9, 2: 1/2, 3: 2/9, 4: 1/36.
can someone help me understand this problem