-1 17/20 and -1 8/10 can be compared easily if we give them common denominators. If we multiply the second fraction by 2, both fractions will have a denominator of 20.
8 * 2 = 16
10 * 2 = 20
New fraction: 16/20
-1 17/20 vs. - 1 16/20
-1 16/20 (-1 8/10) is greater. (For negative numbers, you can tell which one is greater by determining which one is closest to zero.)