Science and Mathematics for Engineering 6th Edition by John Bird

Exercise 12

A formula for the focal length $f$ of a convex lens is: $\frac { 1 } { f } = \frac { 1 } { \mathrm { u } } + \frac { 1 } { \mathrm { v } }$
Evaluate $v$ when $f = 4$ and $u = 20$ .