### Home > PC > Chapter 8 > Lesson 8.2.3 > Problem8-111

8-111.

2P = Pert

$t=\frac{\text{ln}2}{r}$

$r=\frac{R}{100}$

$t=\frac{\text{ln}2}{r} \ \ \ t=\frac{.6931}{\frac{R}{100}} \ \ \text{ Then simplify.}$

We use 70 because it makes the mental arithmetic easier.