Operating at 40 kV , an X-ray tube has an electric current of 10 mA...

The total electrical power supplied to the X-ray tube is given by $P_{in} = VI$.

Substituting the given values:

$P_{in} = 40 \times 10^3 \times 10 \times 10^{-3} = 400 \text{ W}$

This is the total kinetic energy of the electrons striking the target per second.

Since $1\%$ of this energy is converted into X-rays, the total power emitted as X-rays is:

$P_{X} = \dfrac{1}{100} \times P_{in}$

$P_{X} = \dfrac{1}{100} \times 400 = 4 \text{ W}$