Dear

AccipiterQ,

I'm happy to help. I believe the OA given is correct, and does correctly account for exponential growth. First, here's a blog on exponents:

http://magoosh.com/gmat/2012/exponent-p ... -the-gmat/Part of what's tricky is that the start time, with population p, is at t = 1, and the the later time, at t = 5, is FOUR hours later. That's the first tricky thing.

Also, keep in mind, for exponential growth, of course the size of the population doesn't increase linearly, but the exponent on the function

does increase linearly. Thus, at time #1, the population is a P^A, and at time #2, it's at P^B (where B>A), then at a time exactly between those two times, the population will have an exponent exactly between those two values (a.k.a. the average), P^[(A+B)/2]

Part one: Let r be the unknown hourly growth ratio:

t = 1 --> p

t = 2 --> p*r

t = 3 --> p*r*r = p*(r^2)

t = 4 --> p*r*r*r = p*(r^3)

t = 5 --> p*r*r*r*r = p*(r^4)

Set that last value equal to p^2

p*(r^4) = p^2

r^4 = p

r = p^(1/4)

Column #1, choice =

(A)Given that

t = 1 --> p

t = 0 --> p/r = p/[p^(1/4)] = p^(3/4)

Column #2, choice =

(D)Does all this make sense?

Mike

_________________

Mike McGarry

Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)