how much pure acid should be mixed with 2 gallons of a 40% acid solution in order to get a 70% acid solution

Question

doing beginners math and word problems are my weak points.here's a grid (or similar to it)

 

                             the number of gallons X acid strength= amount of acid

pure acid                               X                       100%

 

40% acid solution

 

70% acid solution

needed

Answer 1

2(0.40) + X = 0.7(X+2)

 

0.8 + X = 0.7X + 1.4

 

0.8 + X - 0.7X = 1.4

 

0.8 + 0.3X = 1.4

 

0.3X = 0.6

 

X = 2