A radioactive chemical decays 20 percent each day. If 48 pounds of this chemical remain today, how many pounds of this chemical were present two days ago?
A. 75
B. 72
C. 70
D. 66
E. 60
We first need to go back to the day before. If 48 pounds of this chemical remain, and if that represents a 20 percent decline from the previous day, then the question we need to ask is “80 percent of what is 48?” Algebraically, we would express that as .8x= 48. Dividing 48 by .8 (or 8/10) is the same as multiplying it by 10/8; 48 x (10/8) = 60. Then we need to go back one more day. The question now is “80 percent of what is 60?” In algebraic terms, that would be .8x=60. Dividing 60 by .8 (or 8/10) is the same as multiplying it by 10/8; 60 x (10/8) = 75. The answer is (A).
source: kaptest.com
