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 June 18th, 2018, 02:10 AM #1 Senior Member   Joined: Aug 2014 From: India Posts: 310 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 02:19 AM.
 June 18th, 2018, 02:32 AM #2 Math Team   Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894 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, 02:42 AM   #3
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Quote:
 Originally Posted by Country Boy 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.
You misunderstood. Actual formulae is SI = PRT / 100.

How 1x + 1x = 2x. is obtained?

June 18th, 2018, 05:23 AM   #4
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Quote:
 Originally Posted by Ganesh Ujwal How X + X = 2X step written? I know SI = PRT / 100.
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, 08:12 AM #5 Global Moderator   Joined: Dec 2006 Posts: 19,502 Thanks: 1739 Wasn't Country Boy incorrect? I've not watched the video yet.
 June 18th, 2018, 08:44 AM #6 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,102 Thanks: 906 Country Boy shows 13.5 which is correct: quote from his post: And 3(4.5)= 13.5 years
June 18th, 2018, 03:11 PM   #7
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Quote:
 Originally Posted by Ganesh Ujwal In how much time would the amount become four times of itself? . . . How X + X = 2X step written?
As Country Boy replied "And 3(4.5) = 13.5 years to be multiplied by 8", that has to be incorrect, doesn't it?

It doesn't correspond to either question. What formula gives that answer?

June 18th, 2018, 03:27 PM   #8
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Quote:
 Originally Posted by Country Boy And 3(4.5)= 13.5 years to be multiplied by 8, 4(4.5)= 18.0 years to be multiplied by 16, etc.
I'm sure he means it like this:
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, 08:29 PM #9 Global Moderator   Joined: Dec 2006 Posts: 19,502 Thanks: 1739 What formula did you use?
 June 18th, 2018, 10:31 PM #10 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,102 Thanks: 906 NO specific formula...I did what I showed... What do you mean with "formula"?

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