The sum of the numbers from 1 to 20 inclusive is 220.
The sum of the first 10 counting numbers (1-10) is 51.
The sum of the first six counting numbers (1-6) is 19.
The sum of the first 100 counting numbers (1-100) is 5,001.
1+1+1+1+1+=5 * * * * * The question did not ask for the sum of the first counting number five times! The sum of the first 5 counting numbers is 1+2+3+4+5 = 15. Such sums are known as triangular numbers.
The sum of the first 500 counting numbers (1-500) is 125,001.
1, 2, 3, and 5
1 and 3
105
20 and 1. 20 + 1 = 21 20 - 1 = 19
I assume you mean the first three "counting" (ordinal) numbers, which are 1, 2, and 3. The sum is therefore 1 + 4 + 9 = 14.
(1 + 20) x (20/2) ie 210
There are (not counting 1) three square numbers between 1 and 20.