Describe how you would help Jamal determine the mass of his bottle using the available materials and help Jamal find out whether his bottle qualifies for sale.

The clockwise moment must equal the anticlockwise moment.

$$m_1 d_1 = m_2 d_2$$

where $m_1$ is the known mass (100g), $d_1$ is its distance from the pivot, $m_2$ is the unknown mass of the bottle, and $d_2$ is its distance from the pivot. You can then calculate $m_2$.

$$\boxed{\text{Use a balanced metre rule as a lever, suspending the 100g mass and the bottle on opposite sides.}}$$ $$\boxed{\text{Adjust positions until balanced, measure distances, and apply the principle of moments to find the bottle's mass.}}$$

The principle of moments states that for an object to be in rotational equilibrium, the sum of the clockwise moments about a pivot must equal the sum of the anticlockwise moments about the same pivot.

$$\text{Clockwise Moment} = \text{Anticlockwise Moment}$$ $$m_1 d_1 = m_2 d_2$$

Here, $m_1$ is the known mass, $d_1$ is its distance from the knife edge, $m_2$ is the mass of the bottle, and $d_2$ is its distance from the knife edge.

Given:
$m_1 = 100 \text{ g}$
$d_1 = 15.0 \text{ cm}$
$d_2 = 37.5 \text{ cm}$

$$(100 \text{ g}) \times (15.0 \text{ cm}) = m_2 \times (37.5 \text{ cm})$$

$$1500 \text{ g} \cdot \text{cm} = m_2 \times (37.5 \text{ cm})$$ $$m_2 = \frac{1500 \text{ g} \cdot \text{cm}}{37.5 \text{ cm}}$$ $$m_2 = 40 \text{ g}$$

The entrepreneur buys bottles that weigh more than 30g.
Since $m_2 = 40 \text{ g}$, and $40 \text{ g} > 30 \text{ g}$, Jamal's bottle qualifies for sale.

$$\boxed{\text{The mass of Jamal's bottle is } \textbf{40 g}.}$$ $$\boxed{\text{Yes, the bottle qualifies for sale because } \textbf{40 g > 30 g}.}$$