The work input is the total work done by the machine not the effective work.
The effective work done by a machine is the output work.
The ratio between work output and work input gives you the efficiency of the machine.
Therefore the work input here is,
Work input = Input Force x distance
= 100 x 10 Nm
= 1000 J
Work output = 500 x 1.5 Nm
= 750 J
Efficiency = 750/1000 x 100%
= 75 %