First let's make the denominators equal, so that we can compare the two fractions more easily.
To do this, we have to find a number that both 15 and 9 divide into. The smallest number that this can happen to is 45.
15 x 3 = 45
9 x 5 = 45
So we have to multiply 15 by 3 in order to make it into 45. If we're changing the denominator, the numerator must change too, by the same multiplier.
4 ---> 4 x 3 = 12
--
15 ---> 15 x 3 = 45
4/15 = 12/45
5 ---> 5 x 5 = 25
--
9 ---> 9 x 5 = 45
5/9 = 25/45
To find how many times more rings there are, we divide 25 by 12. As the result of this isn't an integer, we leave the answer as a fraction:
There rings inventory is 25/12 times bigger than the earrings inventory.