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August 11th, 2012, 01:09 AM  #1 
Member Joined: Jul 2010 Posts: 69 Thanks: 0  Inverse Proportion Problem
Hello, I have a problem that I would not have been able to figure out if it were not for tutors. The problem is that after trying so hard to figure this math problem out on my own, I cannot see how it is done. Here we have the word problem of: If y is inversely proportional to 4x + 5 and y = 4 when x = 8 1/2, find x when y = 12. If someone could help me solve this problem I would appreciate it. I also will ask, if you get the problem correct, how you did it. Thanks. 
August 11th, 2012, 01:41 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,131 Thanks: 431 Math Focus: Calculus/ODEs  Re: Inverse Proportion Problem
We are told: y is inversely proportional to 4x + 5 hence, we may state: (1) We are given: y = 4 when x = 8 1/2 = 17/2 Substituting these values for x and y into (1), we obtain: When we have by substitution into (1) and using the value we found for k: Solve for x. First divide both sides by 12: Subtract through by 5: Divide through by 4: 
August 11th, 2012, 11:39 AM  #3 
Member Joined: Jul 2010 Posts: 69 Thanks: 0  Re: Inverse Proportion Problem
MarkFL, You got the answer correct. Where did the 401 come from? Also, how are you using those fraction symbols? 
August 11th, 2012, 01:18 PM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,131 Thanks: 431 Math Focus: Calculus/ODEs  Re: Inverse Proportion Problem I used 40  1 to make the multiplication easier. I am using to write the mathematical expressions. 

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