From the least to the greatest:
9.09, 9.9, 9.990
If you're messed up with sth like this just write the numbers with equal amount of digits:
9.09, 9.90, 9.99 - and now just compare the numbers after the dot :)
Digits 0, 4, 7, 9: 9,740
Digits 0,3,5,9: 0,359