The main supply voltage to a room is $120 \mathrm{V}$. The resistance of the lead wires is $6 \mathrm{\Omega }$. A $60 \mathrm{W}$ bulb is already giving light. What is the decrease in voltage across the bulb when a $240 \mathrm{W}$ heater is switched on in parallel to the bulb?

A battery of internal resistance $4 \mathrm{\Omega }$ is connected to the network of resistances as shown in the figure. In order to deliver maximum power to the network, the value of $R$ in $\mathrm{\Omega }$ should be,

Four resistances carrying a current shown in the figure are immersed in a box containing ice at $0° \mathrm{C}$. How much ice must be put in the box every $10$ $\min$ to keep the average quantity of ice in the box constant? The latent heat of ice is $80 \mathrm{c}\mathrm{a}\mathrm{l} {\mathrm{g}}^{-1}$.