Answer:
20 gallons of the 40% solution, 60 gallons of the 80% solution
Step-by-step explanation:
Let x = the gallons of the 40% solution, and y = the gallons of a 80% solution. The first thing we want to do here is to convert each percentage into decimal form - including the 70% solution mixture.
40% = 0.40,
80% = 0.80, respectively 70% = 0.70
As you can tell, 0.40 is associated with x gallons, 0.80 is associated with y gallons, and the mixture contains 0.70  80 solution, as 0.70 is associated with 80. Therefore we can formulate the following expression,
 80 solution, as 0.70 is associated with 80. Therefore we can formulate the following expression,
0.40x + 0.80y = 0.70  80
 80 
At the same time x + y = 80, as the solution ( mixture ) is present with 80 gallons. Isolating x, x = 80 - y. Let us plug that into our expression, solving for y, following by x gallons.

 ( Multiply either side by 10 )
 ( Multiply either side by 10 )
 ( Expand )
 ( Expand )



 ( Substitute to solve for x )
 ( Substitute to solve for x )

As you can see there are 20 gallons of the 40% solution, and 60 gallons of the 80% solution.