Question 1147269
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If an algebraic solution is not required, this can be solved informally using the fact that all the ages are "close together".<br>
The average age of all four is 24; the average age of the three oldest is 26.  On average, then, the age of each of the three oldest is 2 years above the average.<br>
So all together, the ages of those three is 3*2=6 over the average.  That means the age of the youngest has to be 6 years under the average.<br>
ANSWER: 24-6 = 18