12 + 13 + 14 + ... 51 + 52 + 53 = ?
A. 1361
B. 1362
C. 1363
D. 1364
E. 1365
explanation showed adding in pairs which added to 65 and then y - x + 1 = 53 - 12 + 1 = 42
and then 21 pairs add to 65 and 65 x 21 and looked at the last digit which was 5
MY QUESTION : WHY IS IT 21 PAIRS? How would we know this?![]()
A. 1361
B. 1362
C. 1363
D. 1364
E. 1365
explanation showed adding in pairs which added to 65 and then y - x + 1 = 53 - 12 + 1 = 42
and then 21 pairs add to 65 and 65 x 21 and looked at the last digit which was 5
MY QUESTION : WHY IS IT 21 PAIRS? How would we know this?