New Exercise: Exponential Growth and Decay Word Problems

This exercise addresses [Trello card 461](https://trello.com/card/exponential-growth-word-problems/4d87e664967a0775082939ab/461), and is intended to slot in after the [Word Problem Solving - Exponential Growth and Decay video](https://www.khanacademy.org/math/trigonometry/exponential_and_logarithmic_func/exp_growth_decay/v/word-problem-solving--exponential-growth-and-decay). No knowledge of logarithms required. [Sandcastle link](http://sandcastle.khanacademy.org/media/castles/alopatin:exp-growth/exercises/exponential_growth_and_decay_word_problems.html).

Contains 5 types of problems, including the following 2 from the video:

![exponential-growth-2 169](https://f.cloud.github.com/assets/238458/251751/2cdf95ae-8b72-11e2-88ae-b0059a4a4e31.png)
[sandcastle link](http://sandcastle.khanacademy.org/media/castles/alopatin:exp-growth/exercises/exponential_growth_and_decay_word_problems.html?debug&problem=exponential-growth-2&seed=169)

![exponential-decay-1 118](https://f.cloud.github.com/assets/238458/251748/23de963a-8b72-11e2-8b80-364c7ef6f87a.png)
[sandcastle link](http://sandcastle.khanacademy.org/media/castles/alopatin:exp-growth/exercises/exponential_growth_and_decay_word_problems.html?debug&problem=exponential-decay-1&seed=118)