The power of a motor is given as 40W in the original system of units. We are asked to find the power of the motor in a new system of units where the unit of length is doubled, the mass is halved, and the unit of time is doubled.

To solve this problem, we need to understand how power is related to length, mass, and time.

The formula for power is given as P = W/t, where P is power, W is work done, and t is time. Work done is given by the formula W = F * d, where F is force and d is distance.

Let's analyze each change in the units and how it affects the power of the motor.

1. Length is doubled:
- Doubling the length will result in a change in the distance traveled by the motor.
- Since work done is directly proportional to distance, doubling the distance will double the work done.
- Therefore, the power will also double.

2. Mass is halved:
- Halving the mass will result in a change in the force required by the motor.
- Since work done is directly proportional to force, halving the force will halve the work done.
- Therefore, the power will be halved.

3. Time is doubled:
- Doubling the time will result in a change in the duration of the work done by the motor.
- Since work done is directly proportional to time, doubling the time will double the work done.
- Therefore, the power will also double.

Now, let's combine all the changes:
- The length is doubled, which doubles the power.
- The mass is halved, which halves the power.
- The time is doubled, which doubles the power.
- Doubling the power twice cancels out the halving of the power.
- Therefore, the power of the motor in the new system of units will be the same as the original power, which is 40W.

So, the correct option is 3) 40. The power of the motor in the new system of units remains the same as the original power.