find the sum of all positive numbers from 5 to 1,555 that are divisible by 5
The numbers from 5 to 1555 that are divisible by 5 are: 5, 10, 15, 20, ... 1545, 1550, 1555
which is five times 1, 2, 3, 4, ..., 309, 310, 311.
So, the sum of all positive numbers from 5 to 1,555 that are divisible by 5 is 5 times the sum of all positive numbers from 1 to 311.
The sum of all positive numbers from 1 to n is: (1/2)n(n+1)
The sum of all positive numbers from 1 to 311 is: (1/2)*311*(311 + 1) = 311*156 = 48,516
Answer: 242, 580