There are two separate coils in a heater . If one col is used then it takes 20 minutes to boil a given amount of water completely. But when its other coil is used then it takes 5 minutes to boil the same amount of water. Calculate the time taken to boil the same amount of water using the same source when the two coils are connected (1) in series and (2) in parallel.

Explanation

Let H=quantity of heat required to boil water than
for `1^(st)` coil `H=(V^(2))/(R_(1))(20xx 60)`, for `2^(nd)` coil `H=(V^(2))/(R_(2))(5xx60)`
When coils are connected in series
`H=(V^(2))/((R_(1)+R_(2)))xx(t_( s)xx60) rArr H=(HV^(2)xx(t_(s)xx60))/((V^(2))/(4)xx(40xx60)+(V^(2)(5xx 60))/(4))rArrt_(s)=25` second
When coils are connected in parallel
`H=V^(2)(Deltatxx60)xx((1)/(R_(1))+(1)/(R_(2)))=V^(2)(Deltatxx60)xx[(H)/(V^(2)(20xx60))+(H)/(V^(2)(15xx 60))]rArr Deltat=4` second.