Tim Gan Math
All Years

1990 TYS

Question 7

Question

A circular card is divided into 3 sectors scoring 1, 2, 3 and having angles $135{}^\circ $, $90{}^\circ $, $135{}^\circ $ respectively. On a second circular card, sectors scoring 1, 2, 3, have angles $180{}^\circ $, $90{}^\circ $, $90{}^\circ $ respectively (see diagram). Each card has a pointer pivoted at its centre. After being set in motion, the pointers come to rest independently at random positions. Find the probability that

  1. the score on each card is 1.
  2. the score on at least one of the card is 3.

The random variable $X$ is the larger of the two scores if they are different, and their common value if they are the same. Show that $\text{P}\left( X=2 \right)=\frac{9}{32}$ . Show that $\text{E}\left( X \right)=\frac{75}{32}$ and find $\text{Var}\left( X \right)$.