SOLUTION: Express the average of three sisters in the terms of the age (a) of the firstborn (in years) if the second was born 5 years after the first and the third was born 5 years after the

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Question 904605: Express the average of three sisters in the terms of the age (a) of the firstborn (in years) if the second was born 5 years after the first and the third was born 5 years after the second.
I've tried it a bunch of different ways and can't figure out the correct answer. Please help!
Answer by MathLover1(20849) About Me  (Show Source):
You can put this solution on YOUR website!
the age of the firstborn (in years) :a
the second was born 5 years after the first (five years younger):a-5

and the third was born 5 years after the second:%28a-5%29-5=a-10
the average of three sisters in the terms of the age:%28a%2B%28a-5%29%2B%28a-10%29%29%2F3=%283a-15%29%2F3=3a%2F3-15%2F3=a-5