Javier bought a microwave for $105. the cost was 30% off the original price. what was the price of the microwave before the sale?

The original cost of the microwave before the sale can be calculated by the formula:

D =( 1 – i)P, where D is the discounted price, P is the origanal price and I is the percent mark down

105 = (1- 0.30) *P

P = $ 150 dollar is the original price of the microwave

We have 12 liters of 8% acid solution to which we add an unknown amount of liters (x) of 20% acid solution to make a 16% acid solution.
.08 * 12 +.20x = .16 * (12 +x)
.96 + .20x = 1.92 + .16x
.04x = .96
x = 24 liters of 20% acid solution must be added

Source http://www.1728.org/mixture.htm
see example B