5x1 + 5x2 + 5x3 + 5x4 ..... 5x100 = 5 * (1 + 2 + 3 + 5 .... + 100)

= 5 * (sum of n numbers)

For calculating the sum of n numbers we have the formula

i.e (n * (n + 1)) / 2

apply the formula in the above equation.

where n = 100

so

= 5 * (100 * (100 + 1)) / 2

= 5 * (50 * 101)

= 5 * 50 * 101

= 25250

This the sum of the first 100 positive number which is multiples of 5.

And for average:

= 25250 /100

= 252.50