Correct Answer - b.
`(1)/(f_(1))=(mu-1)[(1)/(R_(1))-(1)/(R_(2))]=(1.5-1)[(1)/(14)-(1)/(oo)]=(0.5)/(14)`
`(1)/(f_(2))=(1.2-1)[(1)/(oo)-(1)/(-14)]`
`(1)/(f_(1))=(0.2)/(14)`
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(0.5)/(14)+(0.2)/(14)=(0.7)/(14)`
`(1)/(v)=(7)/(140)-(1)/(40)=(1)/(20)-(1)/(40)(1)/(v)=(2-1)/(40)`
`v=40cm`