we need to find remainder of $$3^{21}$$ when divided by 5 9 gives a remainder -1 when divided by 5 and hence 10 pairs of 9 can be made from 3^21 and one 3 will be left which will give 3 as the remainder and as we know that remainders are distributive in nature the question can be converted to that find remainder of $$frac{3}{5}$$ which is equal to 3