It is very easy
Let the price of a commodity be $50 and you want to subtract 20% from it
Calculate what is 20%of $50
20% of$50
=0.20*50
=$10
Now subtract this $10 from $50 and you get your answer as $40
You may also get the answer directly in the following way
In question of percentages the whole qty. or amount is always 100%
Suppose you get an discount of30% on $60
What price would you have to pay?
100%-30%=70%
Now,70% of $60
=0.70*$60
=$ 42
So you have to pay $42