Average No of cookies : 820

No of cookies sold on Good days = 980

No of cookies sold on fair days = 640

There are two methods to do it:

1. Allegation method - Here we can consider that we are mixing two things with concentration 980 and 640 and getting final concentration of 640.

Pls see attached doc for reference:

2. Method two is algebraic approach, consider no of good days as X and fair days as Y

So total no of cookies sold would be 980*X + 640*Y

we know average = 820 = Total no of cookies sold/Total no of days

= (980*X + 820*Y)/(X + Y) = 820

On solving this we will get 16X = 18Y

which is X/Y = 18/16 = 9/8

HTH

Attachments

**File comment:** Allegation method

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