If S is the infinite sequence S1 = 6, S2 = 12, ..., Sn = Sn-1 + 6,..., what is the sum of all terms in the set {S13, S14, ..., S28}?
1,800
1,845
1,890
1,968
2,016
1,800
1,845
1,890
1,968
2,016
The answer is 1968 (D)
S13 = 6*13 = 78
S28 = 6*28 = 168
The mean of a sequence with equal intervals (in this case 6) is the average of the min and max. So the mean here is 123.
There are 16 numbers in this sequence (28-13)+1 = 16
So the sum of all terms in the sequence is 16*123 = 1968
