Precalculus (6th Edition) Blitzer

Published by Pearson
ISBN 10: 0-13446-914-3
ISBN 13: 978-0-13446-914-0

Chapter 3 - Section 3.5 - Exponential Growth and Decay; Modeling Data - Exercise Set - Page 507: 46

Answer

$56$ years old.

Work Step by Step

Let $P(x)=70$; we have $\frac{90}{1+271e^{-0.122x}}=70$ and $271e^{-0.122x}=\frac{90}{70}-1\approx0.2857$ Thus $x=-\frac{ln(0.2857/271)}{0.122}\approx56$ years old.
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