To solve these equations, we need to simplify the expressions step by step.

(2/5-3/20)-(9/10-4/5)
= (8/20-3/20)-(18/10-8/10)
= (5/20) - (10/10)
= 1/4 - 1
= -3/4

(13/18-3/8)-(5/8-1/3)
= (104/144-27/144)-(45/72-48/144)
= (77/144) - (135/144)
= -58/144
= -29/72

(19/24-4/15)-(7/10-6/15)
= (285/360-96/360)-(168/240-96/240)
= (189/360)-(72/240)
= 63/120 - 36/120
= 27/120
= 9/40

Therefore, the simplified expressions are: