When a number is divided by 45 , the remainder is 21. What will be the remainder when the number is divided by 15 ?

​When the number is divided by 45, the remainder = 21

When dividing a number by a smaller divisor (here 15), the remainder will be the same as when dividing by a larger number, provided that the remainder from the larger divisor is less than the smaller divisor.

Solution:
No. = 45Q + 21
When divided by 15.
No. = 45Q +21
45Q is divisible by 15.
=> 21 – 15 = 6  

thus, the remainder is 6.