40% of Ron's clothes needs to be washed. 80% of the clothes that don't need to be washed is pure cotton. 70% of all of Ron's clothes are pure cotton, and he has 24 pieces of clothing that don't need to be washed and are not pure cotton. How many pieces of clothing does Ron have that need to be washed and are not pure cotton?
Look at the following sequence of probability tables
We see the answer is 36, choice B.
Let the total =T
Let the pure cotton=C
Let the rest =R
C + R =T
0.6T x 0.8 =0.48T = 96 of 60% are Pure Cotton
0.6T - 0.48T = 0.12T=24 of 60% are NOT Pure Cotton
T =24/0.12 =200 Total pieces of clothings.
0.7*200 =140 Pure Cotton
R =200 -140 =60 pieces that are NOT Pure Cotton.
140 -96 =44 pieces that need washing are Pure Cotton
80(or 40% of 200) - 44 =36 pieces that need wasing are NOT Pure Cotton.