There are four mutually exclusive groups, therefore, the probability of one item being classified into any of the four groups is 1/4 or 0.25.
The expected number of items is calculated as the product of the probability and the total number of observations. For this problem, the probability of an item being classified to group 1 is 0.25 and the total number of observations is 400. Therefore, the expected number of items is
[tex]Expected\:number\:of\:items=0.25\times 400=100[/tex]
The expected number is 100.
