In Cover&Thomas Example 2.1.2 there is an example of the minimum number of binary questions in order to determine the "random variable" $X$: $X=a$ with probability 1/2, $X=b$ with probability 1/4, $X=c$ with probability 1/8 and $X=d$ with probability 1/8.
One can easily see that the maximum number of questions in order to know $X$ are three questions. What I don't understand is that the text states: "The resulting expected number of binary questions required is 1.75". The entropy is precisely this number, but how to understand that three questions, out of 4 possible, is expected to be 1.75?