Question 589535
Hello,

I will call Mr. Black's age is x and Jerry's age is y. 

Mr. Black is three times as old as Jerry: x = 3y

"Nine years ago he was nine times as old as Jerry"

9 years ago, Mr. Black's age was (x - 9) and his son Jerry's age was ( y - 9)

so we have the second equation: (x - 9) = 9(y - 9)

so we have two equations:

x = 3y   
(x - 9) = 9(y - 9)

We will substitute x = 3y into the second equation to find y:

(3y - 9) = 9(y - 9)
3y - 9 = 9y - 81
3y - 9y = 9 - 81
-6y = -72
y = -72/-6
y = 12

And we know x = 3y
so x = 3(12) = 36

Thus Mr. Black's age is 36 years old and Jerry is 12 years old.

Hope it helps you:)
John10