## 165. Math Puzzle $2 / 79$

A motorcycle battery ( $6 \mathrm{~V}$ ) was discharged through a lamp ( $6 \mathrm{~V}, 0.5 \mathrm{~W}$ ). The lamp was on for a total of 48 hours.

How many Ah was the battery charged?

Explanation: Ah is the abbreviation for Ampere-hour, a unit of electrical charge.
When discharging the motorcycle battery, the current $I$ flows for 48 hours,

$$
I=\frac{P}{U}=\frac{0.5 \mathrm{~W}}{6 \mathrm{~V}}=\frac{1}{12} \mathrm{~A}
$$

This corresponds to a charge $Q$ of 4 Ah, because

$$
Q=I \cdot t=\frac{1}{12} \mathrm{~A} \cdot 48 \mathrm{~h}=4 \mathrm{Ah}
$$