It is A: They add energy to the work
1) 15 / 12 = 1.25 ratio
2) to increase acceleration 1.25 times (with same F, or same engine) you have to lower mass 1.25 times
3) 1515/1.25 = 1212 kg
choose A
The greatest amount of inertia can be calculated by the following equation
Kinetic Energy = 0.5 x mass x (velocity)^2
You are trying to find Kinetic Energy.
KE = 0.5 x 30kg x 100
KE = 1,500
Your answer is (2) because when using the equation, the child has the greatest amount of inertia.