well you have...
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
are your first 20 odd positive integers.
the numbers that will give you a remember of 1 when divided by 3 is
1 + 7 + 13 + 19 + 25 + 31 + 37 = 133
Hello, Rimas!
Find the sum of the first twenty odd positive integers
that have a remainder of 1 when divided by 3
The numbers are: .7, 13, 19, 25, . . .
This is an arithmetic sequence with first term a = 7 and common difference d = 6.
The sum of an A.S. is: .Sn .= .½n[2a + (n-1)d]
Therefore: .S20 .= .½(20)[2·7 + 19·6] .= .1280