text saying what you are doing.
Let x be the number of gallons of 100% solution, and y be the number of
gallons of 50% solution of formaldehyde used. Then as there are 15 gallons
x + y =15.
Also the 15 gallons contains 0.8*15 gallons of pure formaldehyde, while
x gallons of the 100% solutions contain x gallons or pure formaldehyde, and
y gallons of 50% solution contain 0.5*y or pure formaldehyde, so:
0.8*15 = x + 0.5*y,
x + 0.5 y = 12
so now we have two equations:
x + y = 15
x + 0.5y =12
which we solve to find that y=6 gallons and x = 9 gallons.