An ideal gas is taken through the cycle $A\to B\to C\to A$, as shown in Fig. If the net heat supplied to the gas in the cycle 15J, the work done by the gas in the process $C\to A$ is _____ J.

From given information  $P_C=70N{m}^{-2}$
$\therefore$  work done  $=\frac{-1}{2}(70+100)\times 1=-85J$

Alternative method :

dQ= du$+$dw  in cyclic process du=0 

dQ= dw     dw= work done in AB +Work done in BC+work done in CA  

15= 100( 2-1) + 0 +Work done in CA

$W_{CA}=-85 joule$