Please help!

The summation of all positive integers less than 400   is   79800     now just subtract the summation of all integers <400 that are divisible by 5  (previous answer).

Use the same procedure in your question about "not divisible by 3".

You should get =64,000.

Sum of natural numbers < 400  =   399 * 400  / 2   = 79800

Sum of natural numbers  < 400 divisible by 5  =  5 [ 1 + 2 + 3 + .....+ 78 + 79  ]  =

5 * [79 * 80 / 2]  = 15800

So

Sum of natural numbers  < 400 not divisible by 5  = 79800 - 15800  =    64000