There are two way sof doing this question:
1st solution is :
6/15 = 2/5
because 6/15 is divisible by 3 and it is done by this form :
3 X 2 = 6
and
3 X 5 = 15
2nd solution is:
6/15 = 2/?
here we suppose that the missing value is x:
6/15 = 2/x
you shift the denominators and then multiply them by the their numerators:
6 * x = 15 * 2
6x = 30
now you divide 6 on both sides:
6x/6 = 30/6
x = 5