## 80. Math Puzzle $1 / 72$

How many lamps labeled $40 \mathrm{~W}, 6 \mathrm{~V}$ can be lit in an apartment if the voltage has dropped to $200 \mathrm{~V}$ and the total current is limited to 6 A by the fuse?
The lamps are connected in series. The internal resistance of each lamp is

$$
R=\frac{U^{2}}{P}=\frac{36}{40}=0.9 \Omega
$$

For $n$ lamps, $200 \mathrm{~V}=n R \cdot 6 \mathrm{~A}=n \cdot 5.4 \mathrm{~V} ; n=37$. Therefore, a maximum of 37 lamps can be lit.