Answer:
Sum of all positive integers less than 400 and divisible by 5 is 15,800.
Step-by-step explanation:
TO FIND : 
The sum of all positive integers less than 400 which are divisible by 5.
The set of positive integers  =  1,2,3, 4,5,6,7,.........,,
  =  1,2,3, 4,5,6,7,.........,, 
Now, the number should be divisible by 5.
SO, the desired set of positive integers =  { 5,10,15,20,.......}
Again the numbers are LESS than 400.
So, the desired set of positive integers =  { 5,10,15,20,....... 385,390, 395}
 
Here, First term a  = 5, common difference d  = 4 and last term an = 395

⇒There are total 79 terms in the series.
So, SUM OF 79 TERMS  = 

Hence, The sum of all positive integers less than 400 which are divisible by 5 is 15,800.