1/5 divided 3=1/15 because

A number being divided by a number would be the same as a number multiplied by the reciprocal of the other number.

Reciprocal: the “flip” of a fraction (ex. the reciprocal of 6/7 is 7/6)

Example:
5/6 / 1/6
= 5/6*6/1
=30/6
=5

We can apply the same to the question 1/5 / 3.

1/5 / 3
=1/5*1/3
=1/15

I hope this helps! Please comment if you have any questions.