How can I solve this problem?

Question

write a function with input and output that takes a decimal number and returns the summation of its bits

Answer 1

so 5 which is 101 sums as 2 right?

Answer 2

so 9 which is 1001 is 5?

Answer 3

so 16 is 5.

Answer 4

John Wells nup, 3 and I can't get the problems' idea

Answer 5

John Wells 4