Two solutions of different concentrations of acid are mixed creating 40 ml of a solution that is 32% acid. one-quarter of the solution is made up of a 20% acid solution. the remaining three-quarters is made up of a solution of unknown concentration, c.
which equation can be used to determine c, the unknown concentration?

Taking the 3 solutions as 3 different terms, we can create an equation as follows:

Solution 1 : 10mL with 20% acid
Solution 2 : 30mL with x% acid
Solution 3 : 40mL with 32% acid

Since solution 1 + solution 2 = solution 3, let us substitute the given values we have:

10(0.2) + 30(x) = 40(0.32)
2 + 30x = 12.8

To solve for the unknown concentration x, we subtract 2 from both sides:
2 + 30x - 2 = 12.8 - 2
30x = 10.8

Dividing both sides by 30:
30x/30 = 10.8/30
x = 0.36

Therefore the unknown solution is 36% acid.