Question

A weight lifter lifts 400 lbs on his fifth attempt, thereby increasing his average by 5 lbs. What is his average after the fifth attempt?

Answer 1

The average μ for his first 4 attempts is: μ = (x + x + x + x)/4 = 4x/4 = x = μ

The average after the fifth lift is (x + x + x + x + 400)/5 = (4x + 400)/5 = x + 5 = μ + 5

So obviously Sx after the 5th lift was 25 lb more than Sx after the first 4.

So the average for the first 4 was 375 lb each.

And his average after the fifth attempt is
375 x 4 =1500
400 x 1= ..400
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So μ = 1900/5 = 380 lb.

Answer: The average after the fifth attempt was 380 lb