Find the focal length of equivalent system shown in Figure.

Explanation

`(1)/(f_(1)) = ((3)/(2) - 1)((1)/(10) + (1)/(10)) = (1)/(2) xx (2)/(10) = (1)/(10)`
`(1)/(f_(2)) = ((6)/(5) - 1)((-1)/(10) - (1)/(20)) = (1)/(5) xx ((-30)/(10xx20)) = (-3)/(100)`
`(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), f = (100)/(13)`