Calculus disaster
posted by Amanda .
Uranium ore has two main isotopes, mostly U238 with just a trace amount of U235. In a sample of Uranium ore, 99.85% of the atoms are U238 atoms and 0.15% are U235 atoms.
Before the Uranium can be used in a nuclear power plant, the proportion of U235 must be increased to 15% (thus reducing the proportion of U238 to 85%). This is done by a process called gas diffusion. The ratio of the masses of these two isotopes is U238 to U235 = 1.013. Each cycle of the gas diffusion process decreases U238 by 1.3%. How many cycles are required to reduce the U238 to 85%?

You want to go from 99.85% U238 to 85%.
Each cycle of the process decreases the U238 fraction by a ratio 10.013 = 0.987. Let the number of processing cycles required be N.
0.850/0.9985 = 0.8513 = 0.987^N
Solve for N.
N = Log0.8513/Log0.987 = 12.3
Call it 13, for a margin of safety
This is not a calculus problem. Precalc, maybe. 
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