Answer
2 gallons of pure acid should be mixed with 2 gallons of a 40% acid solution to get a 70% acid solution.
Work Step by Step
number of gallons × acid strength = amount of acid
x=# of gallons
Gallons of pure acid = x
Gallons of 40% acid = 2
Gallons of 70% acid needed = x + 2
$1x+0.40(2)=0.70(x+2)$
$1x+0.80=0.70x + 1.40$
$1x-0.70x = 1.40-0.80$
$0.30x=0.60$
$x=0.60\div0.30$
$x=2$