The graph between two temperature scales P and Q is shown in the...

From the graph

For scale $P$,
Lower fixed point$= 180 - 150 = 30 ° P$

Upper fixed point$= 180 ° P$

For scale $Q$,
Lower fixed point$= 0 ° Q$

Upper fixed point$= 100 ° Q$

Thus, the relationship between the two scales $P$ and $Q$ is given by

$\dfrac{{t}_{P} - 30}{180 - 30} = \dfrac{{t}_{Q} - 0}{100 - 0}$

$⇒ \dfrac{{t}_{P} - 30}{150} = \dfrac{{t}_{Q}}{100}$