Answer:
1.25 gallons of alcohol
Step-by-step explanation:
Let x represent the amount of alcohol to add to the mix. Then the total amount of alcohol in the mix is ...
0.15×20 + x = 0.20×(20 +x)
3 +x = 4 + 0.2x . . . . simplify
0.8x = 1 . . . . . . . . . . add -3-0.2x
x = 1/0.8 = 1.25 . . . . divide by 0.8
1.25 gallons of alcohol should be added to make 21.25 gallons of 20% alcohol.
