Greetings!
When multiply/dividing fractions, you DON'T find a common denominator. You only need to find a common denominator of two fractions when adding or subtracting.
Multiplying/Dividing Fractions:
3/4*5/8
3*5=15
4*8=36
15/36
Adding/Subtracting Fractions:
1/2+3/10
Find Common Denominator.
5/10+3/10
=8/10
Reduce.
=4/5
Hope this helps.
-Benjamin