Find the focal length of equivalent system shown in Figure.

Answer

B

Explanation

`(1)/(f_(1)) ((3)/(2)-1) ((1)/(10)+ (1)/(10)) = (1)/(10), (1)/(f_(2)) = ((6)/(5)-1)((-1)/(10)-(1)/(20)) = (-3)/(100) "and" (1)/(f_(3)) = ((8)/(5)-1)((1)/(20) + (1)/(20)) = (3)/(50)`
`(1)/(f) = (1)/(f_(1)) + (1)/(f_(2)) + (1)/(f_(3)) = (1)/(10) + (-3)/(100) + (3)/(50) = (100)/(13) = (50x)/(13) implies x=2`