Three equal resistors connected in series across a source of e.m.f. together dissipate 10 W of power. What should be the power dissipated if the same resistors are connected in parallel across the same source of e.m.f.?

90W

Let resistor has a resistance of R each and emf of the cell is $\xi$

$R_s = 3R , P = \frac{\xi^2}{3R} = 10W \Rightarrow \xi^2 = 30R$

$ \text{In Parallel } P' = 3\frac{\xi^2}{R} = 90W$