There are two mutualy exclusive cases here all from the first litter and all from the second litter.

There are 7 rats in litter 1 and 8 rates in litter 2 for a total of 15.

P(both from litter 1)=(7/15)(6/14)

(that is the probability that the first rat selected is from litter 1 is 7/15 times

the probability that the second rat selected (given that the first is from litter 1) is 6/14)

P(both from litter 2)=(8/15)(7/15)

P(both from same litter) = P(both from litter 1)+P(both from litter 2).

RonL