
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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June 18th, 2018, 03:10 AM  #1 
Senior Member Joined: Aug 2014 From: India Posts: 343 Thanks: 1  How X + X = 2X step written?
Question: A certain amount put a simple interest doubles itself after 4.5 years. In how much time would the amount become four times of itself? In this Video I am having trouble in understanding. How X + X = 2X step written? I know SI = PRT / 100. Last edited by Ganesh Ujwal; June 18th, 2018 at 03:19 AM. 
June 18th, 2018, 03:32 AM  #2 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 896 
Perhaps I am not understanding what you are asking but saying that "x+ x= 2x" is just saying that "doubling" something, here x, is just the same as adding it to itself. In any case saying that "the amount doubles in 4.5 years" and then asking "how long does it take to be multiplied by four?" doesn't really require a "formula". 4 is 2 times 2 so "being multiplied by four" is just doubling twice. If it takes 4.5 years to double then it takes 2(4.5)= 9.0 years to be multiplied by 4. (And 3(4.5)= 13.5 years to be multiplied by 8, 4(4.5)= 18.0 years to be multiplied by 16, etc. 
June 18th, 2018, 03:42 AM  #3 
Senior Member Joined: Aug 2014 From: India Posts: 343 Thanks: 1  
June 18th, 2018, 06:23 AM  #4 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,986 Thanks: 995  Ganesh, if an amount doubles, then it becomes 2 times what it was; so if X doubles, then it becomes 2X...clear? That formula means: total interest = principal * rate * time But why do you insist on using it? Country Boy showed you an easier way! And that guy on your attached video talks at over 100 mph! 
June 18th, 2018, 09:12 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 20,310 Thanks: 1980 
Wasn't Country Boy incorrect? I've not watched the video yet.

June 18th, 2018, 09:44 AM  #6 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,986 Thanks: 995 
Country Boy shows 13.5 which is correct: quote from his post: And 3(4.5)= 13.5 years 
June 18th, 2018, 04:11 PM  #7  
Global Moderator Joined: Dec 2006 Posts: 20,310 Thanks: 1980  Quote:
It doesn't correspond to either question. What formula gives that answer?  
June 18th, 2018, 04:27 PM  #8  
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,986 Thanks: 995  Quote:
And 3(4.5) = 13.5 years. If it doubles again, then.... Like 1, 2, 4, 8, 16..... I went at it this way: step 1: 1 grows to 2 in 4.5 periods, so 1/4.5 per period. step 2: 2 grows to 4 in 2/(1/4.5) = 9 periods step 3: 4.5 + 9 = 13.5  
June 18th, 2018, 09:29 PM  #9 
Global Moderator Joined: Dec 2006 Posts: 20,310 Thanks: 1980 
What formula did you use?

June 18th, 2018, 11:31 PM  #10 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,986 Thanks: 995 
NO specific formula...I did what I showed... What do you mean with "formula"? 

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