J2 Prelims Revision Set Problems Involving Lengths of Objects Q1

Four rods of lengths \(1\), \(2\), \(3\) and \(4\) units are placed in a bag from which one rod is selected at random. The probability of selecting a rod of length \(l\) is \(kl\). Find the value of \(k\).

Show that the expected value of \(X\), the length of the selected rod, is \(3\) units and find the variance of \(X\).

After a rod has been selected it is not replaced. The probabilities of selection for each of the three rods remain are in the same ratio as they were before the first selection. A second rod is now selected from the bag. Defining \(Y\) to be the length of the rod and writing \({{P}_{1}}=\mathrm{P}\left( Y=1|X=2 \right)\), \({{P}_{2}}=\mathrm{P}\left( Y=2|X=1 \right)\) show that \(16{{P}_{1}}=9{{P}_{2}}\).

Show also that \(\mathrm{P}\left( X+Y=3 \right)=\frac{17}{360}\).