Task

The objective is to write a function that finds the sum of all positive multiples of 3 or 5 below n.

Show output for n = 1000.

This is is the same as Project Euler problem 1.

Extra credit: do this efficiently for n = 1e20 or higher.




sum35[n_] := 
 Sum[k, {k, 3, n - 1, 3}] + Sum[k, {k, 5, n - 1, 5}] - 
  Sum[k, {k, 15, n - 1, 15}]
sum35[1000]