The student places object P with its centre at the $15.0cm$ mark. Because object P covers the scale under it, explain briefly how to place this object as accurately as possible so that its centre is exactly at the $15.0cm$ mark.

The distance $a$ is the distance between the centre of object P and the pivot. The student places object Q on the other side of the pivot and adjusts its position to balance the rule. The centre of Q is at the $74.5cm$ mark. Determine and state the distance $b$ between the centre of object Q and the pivot.

The experiment is repeated for different positions of object P. The results are shown in Table 1.1:

Using the first and last data points in Table 1.1, calculate the gradient $G$ of the line of best fit, where $a$ is on the $y$-axis and $b$ is on the $x$-axis.

The gradient $G$ of the graph is equal to the ratio $R$ of the masses of P and Q (where $R=\frac{m_P}{m_Q}$). Record the ratio $R$ of the masses. Give your answer to a suitable number of significant figures.