Fifty persons queue up for chest X$-$ray examinations. Suppose there

are four $$positive$$ cases among them. What is the expected number of

$$negative$$ cases before the first positive case is spotted? $[$Hint: think

of the four as partitioning walls for the others. Thus the problem is

equivalent to finding the expected number of tokens in the first box

under $($IV

$)$ of $3\mathrm{.}3.]$