If the current through a bulb changes by $1\%$,what is the percentage change in the power consumed?

$n$ identical bulbs,each designed to consume power $P$ at a certain voltage source,are connected in series across the same source. The total power consumed by them is:

$A$ resistor develops $500 \, J$ of thermal energy in $20 \, s$ when a current of $1.5 \, A$ is passed through it. If the current is increased from $1.5 \, A$ to $3 \, A$,what will be the energy (in $J$) developed in $20 \, s$?

$A$ lamp of $60 \, W, 230 \, V$ is used for $8 \, \text{hours}$. If the cost of $1 \, \text{unit}$ is $1.25 \, \text{Rs}$, what will be the total bill in $Rs$?

$A$ $12 \text{ HP}$ motor has to be operated $8 \text{ hours/day}$. How much will it cost at the rate of $50 \text{ paisa/kWh}$ in $10$ days? (in $Rs.$)

Two electric bulbs of $40 \text{ W}$ each are connected in series. The power consumed by the combination will be . . . . . . . (in $\text{ W}$)