Urn I and Urn II each have 2 red chips and 2 white chips. Two chips are drawn simultaneously from each urn. Let X1 be the number of red chips in the first sample and X2 the number of red chips in the second sample. Find the pdf of X1 + X2

Answer 1

Pdf of X1 = pdf of X2
=(1/2,1/2) 0≤X1≤1, X1∈Z
= 0 otherwise

Pdf of X = X1 and X2
= (1/4, 1/2, 1/4) 0≤X≤2, X∈Z
= 0 otherwise

Answer 2

Please explain?