Step 1: Calculate the rolling radius.
The nominal radius of the rear wheel is 0.5 m, and the rolling radius is 6% less than the nominal radius. Thus,
\[
{Rolling radius} = 0.5 - (6\% \times 0.5) = 0.5 - 0.03 = 0.47 \, {m}.
\]
Step 2: Calculate the theoretical distance for 20 revolutions.
The theoretical distance that the wheel would cover for 20 revolutions, assuming no slip, is:
\[
{Theoretical distance} = 20 \times (2 \pi \times {Rolling radius}) = 20 \times (2 \pi \times 0.47) = 59.04 \, {m}.
\]
Step 3: Calculate the slip.
The slip is the difference between the theoretical distance and the actual distance covered, divided by the theoretical distance and multiplied by 100:
\[
{Slip} = \frac{{Theoretical distance} - {Actual distance}}{{Theoretical distance}} \times 100.
\]
Substitute the values:
\[
{Slip} = \frac{59.04 - 56}{59.04} \times 100 = \frac{3.04}{59.04} \times 100 = 5.00\%.
\]
Thus, the rear wheel slip is 5.00%.