I believe I answered a question very similar to this earlier today. An easy way to find which is the greater is to convert them so that they have the same denominator. 40 would be an easy one.
40 divided by 10 would be 4, so multiply both the 9 and 10 in 9/10 so that you get: 36/40
40 divided by 8 would be 5, so multiply both the 6 and 8 in 6/8 so that you get: 30/40.
36/40 is greater than 30/40, so 9/10 is greater than 6/8.