To solve these subtraction problems, we need to find a common denominator for each pair of fractions and then subtract the numerators.

1) 3/4 - 5/8:
The common denominator for 4 and 8 is 8.
Converting 3/4 to have a denominator of 8: (3/4) * (2/2) = 6/8
Now we can subtract: 6/8 - 5/8 = (6-5)/8 = 1/8

2) 4/9 - 5/18:
The common denominator for 9 and 18 is 18.
Converting 4/9 to have a denominator of 18: (4/9) * (2/2) = 8/18
Now we can subtract: 8/18 - 5/18 = (8-5)/18 = 3/18 = 1/6

3) 13/20 - 2/5:
The common denominator for 20 and 5 is 20.
Converting 2/5 to have a denominator of 20: (2/5) * (4/4) = 8/20
Now we can subtract: 13/20 - 8/20 = (13-8)/20 = 5/20 = 1/4

Therefore, the solutions are:
1) 1/8
2) 1/6
3) 1/4