Percent decrease problem

[dwill]

A radioactive chemical decays 20% each day. If 48 pounds of this chemical remain today, how many pounds of this chemical were present two days ago.

---The professor of my GMAT class teaches tricks instead of actual solutions, he told us to randomly select from one of the answers and work backwards.
What is the correct way to solve this problem if there were no answers to work backwards from?-------

Thanks,

 

[Denis]

48 * 1.2 = 57.6
57.6 * 1.2 = 69.12

 

[stapel]

48 * 1.2 = 57.6
57.6 * 1.2 = 69.12

Actually, if "today" is twenty percent less than "yesterday", then "yesterday" was twenty-five percent more than "today":

. . . . .T = Y - 0.2Y = 0.8Y = (4/5)Y

. . . . .T / (4/5) = [(4/5)Y] / (4/5)

. . . . .(5/4)T = Y

. . . . .5/4 = 1 ¹/₄ = 1.25 = 125%

So the formulation would, I think, be more along the lines of:

. . . . .today: 48
. . . . .yesterday: 1.25(48)
. . . . .day before yesterday: 1.25(1.25(48))

Hope that helps a bit.

Eliz.

 

[dwill]

...Hope that helps a bit.

Thank you.

 

[stapel]

The professor...teaches tricks instead of actual solutions, he told us to randomly select from one of the answers and work backwards.

You might want to have a serious talk with your academic counsellor or somebody in administration, because it doesn't sound as though you're getting the instruction you paid for.

My best wishes to you on the GMAT.

Eliz.