If the boundary of system moves by an infinitesimal amount, the work involved is given by $dw=-P_{ext}dV$
For irreversible process, $w=-P_{ext}\Delta V$ (where $\Delta V=V_f-V_i$ )
For reversible process, $P_{ext}=P_{\mathrm{int}}\pm dP\approx P_{\mathrm{int}}$
So, for reversible isothermal process $w=-nRT\ln \frac{V_f}{V_i}$
2 mole of an ideal gas undergoes isothermal compression along three different paths:
i) reversible compression from $P_i=2$ bar and $V_i=8Lto{P}_f=20bar$
ii) a single stage compression against a constant external pressure of 20 bar, and
iii) a two stage compression consisting initially of compression against a constant external pressure of 10 bar until $P_{gas}=P_{ext},$ followed by compression against a constant pressure of 20 bar until $P_{gas}=P_{ext}.$   

(Use $R=0.08 l.atm.mo{l}^{-1}.K^{-1}$ )