Use the average formula.

We know that average = sum/# of terms

In this case, the average = 55
the # of terms = N + 1
Sum = 50(n) + 90

So:

55 = (50n + 90)/(n + 1)

55(n + 1) = 50n + 90

55n + 55 = 50n + 90

5n = 35

n = 7

OK I UNDERSTAND THE MATH AND NOW KNOW WHERE I WENT WRONG. I DIDNT TRANSLATE CORRECTLY.. DO YOU KNOW OF A PLACE TO GO TO IN ORDER TO GET BEEFED UP ON MATH TRANSLATIONS?

Based on the other questions that you've posted, I really suggest taking a course (if one is available in your area) or hiring a tutor.

These boards are a great way to supplement a course or for people who are close to their goals to make it on their own, but if you need to make a LOT of progress then you need some extensive resources and access to live help.

its given 90 units per day increases the avg by 55 (50+5).therfore it inadvertly means the 40 extra units produced on that particular day was splitted into 5 units for the rest of the days .hence 40/5 = 8 ( which includes the last day with produced 90 units ) Therefore excluding it ,it wud be 7 days....

Thanks
Senthil