84 IS THE 12TH MULTIPLE OF 7. (12 X 7 = 84) 140 IS THE 20TH MULTIP...
18. 84 is the 12th multiple of 7. (12 x 7 = 84)
140 is the 20th multiple of 7.
The question is asking us to sum the 12th through the 20th
multiples of 7.
The sum of a set = (the mean of the set) x (the number of terms in
the set)
There are 9 terms in the set: 20th - 12th + 1 = 8 + 1 = 9
The mean of the set = (the first term + the last term) divided by 2:
(84 + 140)/2 = 112
The sum of this set = 112 x 9 = 1008
Alternatively, one could list all nine terms in this set (84, 91, 98 ...