The 20% acid liquid solution is produced by adding a gallons

[Max@Math Revolution]

[GMAT math practice question]

The 20% acid liquid solution is produced by adding a gallons of 10% acid liquid solution to b gallons of 50% acid liquid solution. To get 10 gallons of 20% acid liquid solution, how many gallons of 50% acid liquid solution are needed?

A. 1
B. 1.5
C. 2
D. 2.5
E. 3

[Max@Math Revolution]

=>

We have a + b = 10.

( 0.1a + 0.5b ) / 10 = 0.2
=> 0.1a + 0.5b = 2
=> 0.1(10-b) + 0.5b = 2
=> 1 - 0.1b + 0.5b = 2
=> 0.4b = 1
=> b = 2.5

Therefore, D is the answer.

Answer: D