Bacteria such as V. cholerae are known to follow an exponential growth curve rate, and will double their number every 15 minutes. Fortunately, anti-bacterial hand wash can kill 99.9% of bacteria on a surface. If a colony of 500 V. cholerae cells are left alone for 2 hours, then anti-bacterial handwash is applied thoroughly, how many bacterial cells are left?

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Correct Answer: **C.** *128 cells*

If the bacteria double every 15 minutes, then that means over the course of 2 hours, they will double 8 times. 500 x 2^{8} = 128,000 cells on the surface. Then we apply the anti bacterial wash, which kills 99.9% of cells, meaning 0.1%, or 0.001 of the cells remain. 128,000 x 0.001 = 128 cells.

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