A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?
a. 3/40,000
b. 1/3,600
c. 9/2,000
d. 1/60
e. 1/15
According to the OG 11th edition, the answer is the probability of selecting sibling pairs in the junior class multiplied by the probability of selecting a match in the senior class. 60/1,000 x 1/800 = 3/40,000.
My question is why isn't the answer:
the probability of selecting sibling pairs in the junior class AND the probability of selecting a match in the senior class
OR
the probability of selecting a sibling a pair from the senior class AND the probability of selecting a match in the junior class.
(60/1,000 x 1/800) + (60/800 x 1/1,000) = 3/20,000.
a. 3/40,000
b. 1/3,600
c. 9/2,000
d. 1/60
e. 1/15
According to the OG 11th edition, the answer is the probability of selecting sibling pairs in the junior class multiplied by the probability of selecting a match in the senior class. 60/1,000 x 1/800 = 3/40,000.
My question is why isn't the answer:
the probability of selecting sibling pairs in the junior class AND the probability of selecting a match in the senior class
OR
the probability of selecting a sibling a pair from the senior class AND the probability of selecting a match in the junior class.
(60/1,000 x 1/800) + (60/800 x 1/1,000) = 3/20,000.
