A student measures the time of flight of a projectile five times...

First, calculate the mean value of the observations, which represents the most probable true value:
$t_{mean} = \dfrac{4.6 + 4.8 + 5.0 + 5.2 + 5.4}{5} = \dfrac{25.0}{5} = 5.0\text{ s}$.

Next, find the absolute errors in each measurement by taking the magnitude of the difference between each reading and the mean value:
$\Delta t_1 = |4.6 - 5.0| = 0.4\text{ s}$
$\Delta t_2 = |4.8 - 5.0| = 0.2\text{ s}$
$\Delta t_3 = |5.0 - 5.0| = 0.0\text{ s}$
$\Delta t_4 = |5.2 - 5.0| = 0.2\text{ s}$
$\Delta t_5 = |5.4 - 5.0| = 0.4\text{ s}$

The mean absolute error is the arithmetic mean of these absolute errors:
$\Delta t_{mean} = \dfrac{0.4 + 0.2 + 0.0 + 0.2 + 0.4}{5} = \dfrac{1.2}{5} = 0.24\text{ s}$.
Absolute errors are magnitudes and are always positive, so they do not algebraically cancel out. Thus, the statement claiming the mean absolute error is zero is incorrect.

The fractional error is given by:
$\text{Fractional error} = \dfrac{\Delta t_{mean}}{t_{mean}} = \dfrac{0.24}{5.0} = 0.048$.

Answer: The mean absolute error is zero because positive and negative deviations cancel each other out.