3 = 3 + 0*5

8 = 3 + 1*5

13 = 3 + 2*5

18 = 3 + 3*5

.....

48 = 3 + 9*5

If sum the left and the right sides of the equations we have:

3 + 8 + 13 + 18 + ... + 48 = 3*10 + 5(1 + 2 + 3 + ... + 9) = 30 + 5*(1 + 9)*(9/2) = 255