Question

what should be added and subtracted from dividend so that dividend should be exactly divisible by divisor

Answer 1

Just subtract reminder from dividend. And u will have exactly divisible number by divisor.
Ex 16/5 in this case reminder is 1
Subtract it from 16 will get 15.
Which is divisible by 5

OR

Add the difference of divisor and 
Reminder to dividend.

Ex 
16/5 
Reminder is 1
Subtract it from 5 will get 4
Add this to 16 will get 20 
Which is divisible by 5.