Solution :
Number of machines => $36 \qquad x$
Number of cars produced => $54 \qquad 81 $
$\qquad= 36 \times 54 = x \times 81$
=>$\qquad= \large\frac{36 \times 54}{81}$$=x$
=>$ \large\frac{4 \times 54}{9} $$=x$
$x= \large\frac{216}{9} $
$x=24$
Answer : 24