posted by JULIE on .
THE TOTAL AGE OF GROUP OF PARENTS IS 342.A NEW PARENT AGED 48 JOINS THE GROUP AND THE AVERAGE RISES BY 1. WHAT IS THE ORIGINAL AVERAGE.
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let there be n parents
The average age at first is 342/n
After adding 1 parent and 48 years, the new average is 390/(n+1)
342/n + 1 = 390/(n+1)
342(n+1) + n(n+1) = 390n
342n + 342 + n + n^2 - 390n = 0
n^2 - 47n + 342 = 0
342 = 2*171 = 2*3*57 = 2*3*3*19
(n-38)(n-9) = 0
so there were either
38 parents average 9 years old (not likely), or
9 parents average 38 years old
when a 48-year old joins, there are now 10 parents avg age 39