Bunuel wrote:

The four integers a, b, c, and d are such that a < b < c < d, d = 7a, and b - a = c - b = d - c. What is the average (arithmetic mean) of the four integers?

(1) c = 25

(2) c/a = 5

b - a = c - b MEANS b is average of a and c..

c - b = d - c MEANS c is average of b and d..

this means a, b,c and d are equally spaced..

since d=7a, terms are \(a,3a,5a,7a\)......

if we know any term, we can get all terms..

average = 4a

(1) c = 25

5a = 25....a=5

average = 4*5=20

sufficient

(2) c/a = 5

nothing new we kcow this info from statement itself

c/a = 5a/a = 5

insuff

A

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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372

2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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