# Differential Equations : Radioactive Decay

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The rate of decay of a radioactive substance is proportional to the amount of the substance present. Two years ago there were 5 grams of substance. Now there are 4 grams. How many grams will there be 4 years from now?

A. 16/25 B. 2 C. 64/25

D. 16/5 E. 25/4

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##### Solution Summary

Rate of decay is used to calculate the amount of a radioactive substance after a given time has elapsed. The solution is detailed and well presented. The response was given a rating of "5" by the student who originally posted the question.

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The answer is C.

Suppose the amount of the radioactive substance is y and the time it t years.

The rate of decay is ...

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