SOLUTION: The quality control department in a clothing factory found an average of two rejects per batch in the first 15 batches checked. After a further 15 batches had been through quality

Question 828906: The quality control department in a clothing factory found an average of two rejects per batch in the first 15 batches checked. After a further 15 batches had been through quality control, the average number of rejects per batch had risen to 7. What was the average number of rejects per batch in the last 15 batches only
Answer by KMST(5328) (Show Source): You can put this solution on YOUR website!
THE USUAL WAY:
The average for the first 15 batches is the sum of rejects for those 15 batches divided by 15.
The average for all 30 batches is the sum of rejects for those 30 batches divided by 30.
The average for the second/last set of 15 batches is the sum of rejects for those 15 batches divided by 15, and is what you want to find.

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THE FAST WAY (useful when you need a quick answer, but do not need to show your work):
The average for a larger group made of two groups with the same number of elements (such as a group of 30 batches made of two sets of 15 batches each) is the average of the averages.
And the average of 2 numbers, is the number at the same distance from both numbers.
So if averaging 2 and the average you want to find you would get 7, the average you are looking for is also
units away from 5.
During a multiple choice test, you can do these calculations in your head quickly, write the answer and go to the next question.
So the other average is
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