A gasoline distributor has a 20,000-gal tank to store a gasohol mix. (CONTINUED)

Question

A gasoline distributor has a 20,000-gal tank to store a gasohol mix. The tank already contains 12,000 gal of gasohol that contains 4% ethanal by volume. How many gallons of ethanol must be added to the tank to produce a mixture that is 10% ethanol? 

Answer 1

4% = 12000

10% =?

cross multiplying, 

we get: (12000/4)*10

simplifying: the answer is 30000 gallons of ethanol.