# How to formulate exponential growth?

Here's my question:

A rumour spreads exponentially through a college. 100 people have heard it by noon, 200 by 1pm. How many people have heard it

a) by 3pm b) 12.30pm c)1.45pm

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do you know the equation for exponential growth? –  yiyi May 13 '12 at 17:11
In other words, it doubles every hour. That should get you 3:00 PM easily. –  Michael Hardy May 13 '12 at 17:33

(a) the number doubles every hour, i.e. multiplies by $2$
(b) the number multiplies by $\sqrt{2}$ every half-hour
(c) the number multiplies by $2^\frac14$ every quarter of an hour