2. a chemist wants to make 4 liters of a 7% acid solution by mixing a 10% acid solution and a 4% acid solution. how many liters of each solution should the chemist use? write your answer as a complete sentence be sure to: (a) define your variable and expressions for the quantities. (b) write an equation that models the problem. (c) solve the equation. (d) state the answer in a complete sentence and explain

( * = multiplication/multiply/times ) 
Let x - liters of 10% acid solution needed
4-x = liters of 4% acid solution needed

.1x + .04*(4-x)
= .07
           4

.1x + .04*(4-x) = 4*.07
.1x + .16 - .04x = .28
.06x = .12
x=2
4-x=2

2 liters of 10% acid solution are needed
2 liters of 4% acid solution are needed

If you're still uncertain, we can check our work as so: 

.1x + .04*(4-x)
= .07
            4

.1 * 2 + .04 * (4-2)
= .07
             4

.2 + .08
= .07
     4

.28 = .07 * 4
.28 = .28 

Hope I helped.