Let a subscript n be the remainder when n is divided by 5.

a)find a subscript 11 and a subscript 400
b) if a subscript m =3, find a subscript (m +4)

what do you get? Just do the divisions and find the remainder. dividing by 5 is particularly easy, since numbers ending in 0-4 and 5-9 have remainders 0-4.