How do you write 15 over 65 in simplest form?

15/65

By cancelling down/reduction.

We note, in this case, that both numbers end in '5'.

So '5' is a common factor; 5 x 3 = 15 and 5 x 13 = 65

So cancel down/reduce by '5'

15/65 = 3/13

Since both '3' & '13' are prime numbers this will NOT reduce any further.

Hence

15/65 in its simplest form is 3/13.

NB Any number, however large or small, that ends in '5' or '0' , has '5' as a common factor. Refer to your '5' times multiplication table.