The average age of the 5 trustees of a temple trust is 40 years. The youngest trustee is 30 years old, and no two trustees are the same whole number of years old. What is the greatest age the eldest trustee can be?

The five ages add to $5\times 40=200$ years. To leave the eldest as old as possible, the other four must be as young as the rules allow: the youngest is 30, and the remaining three must be different whole numbers above 30, so 31, 32 and 33. Those four add to 126, leaving 200 - 126 = 74 for the eldest. 80 packs all four of the others at 30, which breaks the condition that no two share an age. 50 mirrors the youngest's 10 years below the average as 10 years above it, a symmetry the question never claims. 170 takes only the youngest out of the total and forgets the other three altogether.