The first thing we have to do is find 20% of 20. To do this we cross multiply :
(20 / 100 ) * (20 / 1)
Then we simplify
(20 / 5) * (1 / 1)
( 4 ) * ( 1 ) =
4
So we get four. Now remember this is not our final answer. We have to subtract it from the total which is 20. We subtract it because since the price is discounted, it will go down. Also because we just found 20% OF 20 , not 20 minus 20% of twenty.
Since 12 out of 30 marbles were black, you can assume that the percentage of black marbles in the bag is 40% (Ex: 12/30=.4) The probability that the marble you pick is black is equal to the percentage of black marbles in the bag. 40% is equal to 2/5.
