Dr. w needs 1600 ml of 70%-ethanol solution. he has two solutions of ethanol. solution a is 60% ethanol and solution b is 75% ethanol. how much of solution a and solution b does he need to mix together in order to get 1600 ml of 70%-ethanol solution?

We need 1,600 ml of 70% ethanol.
x is ml of 60% and
y is ml of 75%

A)  x + y = 1,600
B)  .60x + .75y = .70 * 1,600

Multiplying equation A by -.60
A) -.60 x -.60y = -960 Then adding this to equation B
B)  .60x + .75y = .70 * 1,600
.15y = -960 + 1,120
.15y = 160
y  (75%) = 1,066.66 ml
x (60%) = 533.33 ml

1.17+3.99a. you combine the terms with a to create 3.99 and the 1.17 stays because it has no like terms or a variable