What is the sum of the first 102 counting numbers?

Answer 1

If you want to sum up numbers 1 to N, you can do it this way. Sum = (N+1)*(N/2).

The reason this works: say you want to sum up 1 through 10. You have 10+1, 9+2, ..., 6+5. Each of these equals 11 [N+1]. You added 11 five times (N/2). Sum of 1 to 102 = (102+1)*(102/2) = 103*51 = 5253. You can check it by summing up the numbers with a spreadsheet.