Question 699445
We have three equations based on the average of two scores is one-half the sum of the two scores, or the sum is twice the average.
(1) S1 + S2 = 2*50
(2) S2 + S3 = 2*75
(3) S3 + S4 = 2*70
Now to solve, first substitute S2 of (2) into (1) and get
(4) S1 + (150 - S3) = 100 or
(5) S1 = S3 -50
Now substitute S3 of (3) into (5) and get
(6) S1 = (140 - S4) - 50 or
(7) S1 + S4 = 90 giving us the average of S1 and S4 
(8) (S1+S4)/2 = 90/2 or 45
The answer is choice a)