5) A wheel with a diameter of $5 \mathrm{~cm}$ is connected to another wheel via a transmission belt. The first wheel rotates at 1000 revolutions per minute. What diameter should the second wheel have to rotate at 200 revolutions per minute?
(A) $20 \mathrm{~cm}$
(B) $25 \mathrm{~cm}$
(C) $27 \mathrm{~cm}$
(D) $50 \mathrm{~cm}$
(E) it depends on the distance between the axes of the wheels.
5) The answer is (B). The ratio between the diameters is inversely proportional to the rotational speed of the wheels. Indicating with $d$ the diameter of the second pulley, we will have that

$$
100 \frac{\text { revolutions }}{\text { minute }} \times 5 \mathrm{~cm}=200 \frac{\text { revolutions }}{\text { minute }} \times d,
$$

from which $d=25 \mathrm{~cm}$.