Tube length( \((l)=\) Distance between lenses \(=v_{o}+f_{e}\) For objective lens,

\({u_o} =  - 200{\text{cm}},{v_o} = ?\)

\(\frac{1}{v} - \frac{1}{{{u_o}}} = \frac{1}{{{f_o}}}\)  or   \(\frac{1}{{{v_o}}} - \frac{1}{{ - 200}} = \frac{1}{{40}}\)

or \(\frac{1}{{{v_0}}} = \frac{1}{{40}} - \frac{1}{{200}} = \frac{4}{{200}}\)  \(\therefore {v_0} = 50\,{\text{cm}}\)

\(\therefore \quad l=50+4=54\, \mathrm{cm}\)