To solve this problem, we need to first simplify each of the expressions within the parentheses:

(2/5 - 3/20) = (8/20 - 3/20) = 5/20

(9/10 - 4/5) = (9/10 - 8/10) = 1/10

Now, we subtract these simplified expressions:

(5/20) - (1/10) = (5/20) - (2/20) = 3/20

Therefore, the answer is 3/20.