Carcass wrote:
A box at a yard sale contains 3 different china dinner sets, each consisting of 5 plates. A customer will randomly select 2 plates to check for defects. What is the probability that the 2 plates selected will be from the same dinner set?
A. 27
B. 25
C. 23
D. 56
E. 32
Solution:
Selecting 2 plates from all the plates ( 3 different sets and each having 5 plates = 3x5 = 15) => 15c2 = (15x14)/2 = 15 x 7
Selecting 2 plates from same set = Selecting 1 set from 3 sets (3c1) and Selected 2 plates from the set selected (5c2).
i.e; 3c1 x 5c2 = 30
Probability = (3c1 x 5c2) / 15c2
= 30/ (15 x 7)
= 2/7