Question : The average weight of a group of 20 persons is 40 kg. The average weight of another group of 40 persons is K kg. If the average weight of all the 60 persons is 60 kg, then what is the value of K?
Option 1: 80
Option 2: 60
Option 3: 40
Option 4: 70
Correct Answer: 70
Solution :
Now, using the average formula, the total weight of all 60 persons is 60 × 60 kg.
⇒ (20 × 40) + (40 × K) = 60 × 60
⇒ 800 + 40K = 3600
⇒ 40K = 3600 – 800 = 2800
⇒ K = $\frac{2800}{40}$
⇒ K = 70
Hence, the correct answer is 70.
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