An electric kettle has tow heating coils. When one of the coils connected to an AC source, the water in the kettle boils in `10` min. when the other coil is used the water boils in `40` min. if both the coils are connected in parallel, the time taken by the same quantity of water of boil will be

Options

A. 8 min
B. 4 min
C. 25 min
D. 15 min

Answer

A

Explanation

`Q=V^2/R_1xxt_1=V^2/R_2xxt_2=V^2/Rxxt`
`1/R=1/R_1+1/R_2rArr Q/(V^2t)=Q/(v^2t_1)+Q/(V^2t_2)`
`rArr 1/t=1/t_1=1/t_2 rArr t=(t_1t_2)/(t_1+t_2)`
`=(10xx40)/(10+40)=`8 min