PROBLEM:

A generation is about one-third of a lifetime. Approximately how many generations have passed since the year 0 AD?

SOLUTION:

We are looking for generations passed in history. The general assumptions are:

• There are 10¹¹ seconds in 1 history
• In 1 generation, there is 1/3 of a lifetime.
• In half a lifetime, there are 10⁹ seconds

Therefore, the number of generations passed is

\begin{align*}
1\ \text{history} & =1\:\text{history}\times \frac{10^{11}\:\text{sec}}{1\:\text{history}}\times \frac{1\:\text{generation}}{\frac{1}{3}\:\text{lifetime}}\times \frac{0.5\:\text{lifetime}}{10^9\:\text{sec}} \\
& =150\:\text{generations} \ \qquad \ \color{DarkOrange} \left( \text{Answer} \right)
\end{align*}