# How many gallons of a 15% acid solution must be mixed with 5 gallons of a 20% acid solution to make a 16% acid solution?

$20$ gallons
$0.15 x + 0.2 \times 5 = 0.16 \left(x + 5\right)$
$0.01 x = 0.2$
$x = 20$ gallons