Answer:
84.4 %
Step-by-step explanation:
Mechanical efficiency = output work/input work × 100 %
output work = 432 J of work for the bike to turn the gears
input work = 512 J of work to ride.
Mechanical efficiency = 432 J/512 J × 100 %
= 0.844 × 100%
= 84.4 %