You can calculate the expected age as the sum of individual ages weighted (multiplied) by the corresponding fraction:
expected age: 1/4* 5 + 1/2*6 + 1/8*7 + 1/8*9 = 5/4 + 6/2 + 7/8 + 9/8 =
50/8 = 6.25
the answer is, the expected age is 6.25 years
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