Cyclist M leaves point P at 12 noon and travels at constant velocity

When two bodies move in same direction, their relative speed = difference of their speeds
Let velocity of M be m and let velocity of N be n. Their relative speed = n-m
First M moves for 2 hours, let distance covered be D, Then time taken to overtake, t = D/(n-m)

Here N overtakes M in two hours. So D/(n-m) = 2

Statement 1. m=20 mph. So, D = 20*2 = 40 miles So,
40/(n-20) = 2. n = 40 mph. Sufficient

Statement 2. N overtook M at 4 pm and at that time, both had travelled 60 miles.
M covers 60 miles from 12 to 4 pm. So m=60/4 = 15 mph
N covers 60 miles from 2 pm to 4 pm. So n = 60/2 = 30 mph.
Sufficient

Hence D answer

(Though I am confused because answers are coming different from the two statements. In GMAT, when the answer is 'D', both statements should give the same answer. Maybe I am missing something)