# Math

An isotope of sodium has a half-life of 20 hours. Suppose an initial sample of this isotope has mass 10 grams. The amount of the isotope (in grams) remaining after t hours is given by:
(Note: We're assuming exponential decay here. Use the exponential decay function)
A. 10(.965936329)^t
B. 10(1.035264924)^t
C. 10e^(-.965936329t)
D. Both A and C
E. Both B and C

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1. a(t) = a(0) e^-kt
when t = 20 , a(20) = a(0) /2
so
0.5 = e*-20 k
-20 k = ln 0.5 = -.693
k = .03466
so
a(t) = a(0) e^-.03466 t

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