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pattern:
common difference is
1st term:
2nd term:
3rd term:
4th term:
5th term:
6th term:
7th term:
and so on
They want you find the sum of an arithmetic progression.
First term is 75; last term is 15; common difference is 3.
The sum is the product of the mean by the number of terms.
The mean is = = 45.
The number of gaps = = = 20.
The number of terms is 20+1 = 21.
The sum is mean * the number of terms = 21 * 45 = 945. ANSWER
