Question 589535
Problem:
Mr. black is three times as old as his son Jerry. Nine years ago he was nine times as old as Jerry. 
Answer supose to be:
3X-9=9(x-9) 
How do they come to this solution?
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Equations based on the problem description.
b = 3j
(b-9) = 9(j-9)
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Substitute for "b" and solve for "j":
(3j-9) = 9(j-9)
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That is the equation you are asking about.
I used "j" to represent Jerry's current age;
the book uses "x" for the same representation.
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Cheers,
Stan H.