Answer: 200N
Explanation: Mechanical advantage is the ratio of how the machine transforms the force that you apply in it.
Ma = Fb/Fa
Where Fb is the force that the machine does and Fa is the force that you do to the machine.
If the mechanical advantage is 15, this means that the force that you apply at the machine is multiplied by 15.
If we want to lift an object that weighs 3000N, the minimal force that we need to apply is also 3000N. so we need to solve the equation:
Fb = 15*Fa = 3000N
Fa = 3000N/15 = 200N
So the force that we need to apply is 200N