It is believed that identical twins have similar IQ levels. Thirty pairs of identical twins participate in an IQ test and their results are recorded. There are no tied or zero ranks. The sum of positive ranks is 272, and the sum of negative ranks is 193.

a. Find E(T').

b. Find Var(T').

c. Using a suitable approximation, test, at the 5% significance level, whether there is a difference between the IQ scores of a set of identical twins.