GMAT Club Official Solution:At a climbing gym, 4 junior climbers and 3 instructors were weighed for harness fitting. If the average (arithmetic mean) weight of all 7 people was 55 kilograms and the heaviest instructor weighed 80 kilograms, was the average (arithmetic mean) weight of the 4 junior climbers less than 53 kilograms?The total weight of all 7 people was:
7 * 55 = 385 kilograms
The question asks whether the total weight of the 4 junior climbers was less than:
4 * 53 = 212 kilograms
(1) The average weight of the other two instructors was 56 kilograms.
So the total weight of those two instructors was:
2 * 56 = 112 kilograms
The heaviest instructor weighed 80 kilograms, so the total weight of the 3 instructors was:
80 + 112 = 192 kilograms
Therefore, the total weight of the 4 junior climbers was:
385 - 192 = 193 kilograms
Since 193 < 212, the average weight of the junior climbers was less than 53 kilograms.
Sufficient.
(2) The three lightest people in the group were junior climbers, each weighing 47 kilograms.
Let x be the weight of the fourth junior climber, and let y and z be the weights of the other two instructors.
Then:
47 + 47 + 47 + x + y + z + 80 = 385
x + y + z = 164
If the average weight of the 4 junior climbers were not less than 53 kilograms, then their total weight would be at least 212 kilograms:
47 + 47 + 47 + x >= 212
x >= 71
Then:
y + z <= 164 - 71 = 93
But since the three 47-kilogram junior climbers were the lightest people, each of the other two instructors must weigh at least 47 kilograms. So:
y + z >= 47 + 47 = 94
This is impossible.
Therefore, the average weight of the junior climbers must be less than 53 kilograms.
Sufficient.
Answer: D.