The sum of the weights of the three adults is 140 x 3 = <<140*3=420>>420 pounds.
The sum of the weight of the two children is 64 x 2 = <<64*2=128>>128 pounds.
So the total weight of the 5 people who are in the elevator is 420 + 128 = <<420+128=548>>548 pounds.
This would mean that the next person's weight must not exceed 600 - 548 = <<600-548=52>>52 pounds.
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