Paper 1 · Non-calculator

A histogram shows waiting times $t$ minutes.

$\begin{array}{|c|c|c|} \hline \text{Interval} & \text{Frequency} & \text{Frequency density} \\ \hline 0 < t \le 10 & 20 & 2.0 \\ \hline 10 < t \le 20 & 35 & 3.5 \\ \hline 20 < t \le 50 & 45 & ? \\ \hline \end{array}$

Calculate the height of the bar for the interval $20 < t \le 50$. [3]