Calculating Work Done by a Batter on a Baseball

[darkley]

Homework Statement

A baseball with a mass of 1kg is thrown with a velocity of -5 m/s. A batter hits the baseball and sends the ball into the outfield with a velocity of 20 m/s. What is the amount of work done by the batter?

Homework Equations

$$W = \Delta KE = KE_{f} - KE_{i}$$
$$KE = \frac{1}{2} m v^2$$

The Attempt at a Solution

After substituting the values into the formula for kinetic energy, I determined that the final kinetic energy of the baseball is 200 joules and the initial kinetic of the ball is 12.5 joules. Would I then simply subtract 12.5 joules from 200 joules to arrive at my final answer?
My teacher insists that the two values need to be added in order to arrive at the work done, and he tells me that even though the work done to change the ball's velocity from -5 m/s to 0 m/s is negative, the magnitudes of the values need to be added to find the total work. Could anybody shed some light on this?

 

[SammyS]

It's hard to argue successfully with a teacher.

However, you are correct! Your analysis spot-on (absolutely right).

Work is a scalar property.

Sam Snyder, PhD in physics, if that helps convince him.

 

[Dragynfyre]

I think your teacher is confusing work with impulse.

 

[ashishsinghal]

Your method is totally fine.

 

[darkley]

Thank you all for your help!