
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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July 16th, 2019, 07:47 AM  #1 
Senior Member Joined: Aug 2014 From: India Posts: 458 Thanks: 1  How he got 25 & 31?
A sum of Rs 800 amounts to Rs 920 in 3 years at SI. if the interest rate is increased by 3%, it would amount to how much? My friend told amazing shortcut: 5%...8% 25==800 31==992 How he got 25 & 31? 
July 16th, 2019, 09:38 AM  #2 
Senior Member Joined: Jun 2019 From: USA Posts: 120 Thanks: 40 
$\displaystyle 1 + (0.08 )(3) = 1.24 = 124/100 = 62/50 = 31/25$ $\displaystyle (31/25)(Rs 800) = Rs 992$ 
July 16th, 2019, 10:04 AM  #3 
Senior Member Joined: Aug 2014 From: India Posts: 458 Thanks: 1 
How you got $\displaystyle 1 + (0.0 8 )3$?

July 16th, 2019, 11:03 AM  #4 
Senior Member Joined: Jun 2019 From: USA Posts: 120 Thanks: 40 
Rs 120 in 3 years is Rs 40 per year. 40/800 = 5 %. Increasing the interest rate to 8 % means in three years (simple interest) you would gain 3*8 % = 24 % of the original amount in interest. Adding 24 % is the same as multiplying by 1.24. If you want, $\displaystyle 100 \% + (\frac{8 \%}{year}) \cdot (3~years) = 124 \%$ 
July 16th, 2019, 07:18 PM  #5  
Senior Member Joined: Aug 2014 From: India Posts: 458 Thanks: 1  This part is OK and understood. Quote:
Actual formula for $\displaystyle Amount = SI + P$. Last edited by Ganesh Ujwal; July 16th, 2019 at 07:28 PM.  
July 17th, 2019, 12:02 AM  #6 
Senior Member Joined: Apr 2014 From: UK Posts: 953 Thanks: 340 
I did it this way: 920 + 0.03*3*800 = 992 
July 17th, 2019, 12:22 PM  #7 
Global Moderator Joined: Dec 2006 Posts: 20,921 Thanks: 2203 
Amount = SI + P implies Amount/P = 1 + SI / P = 100% + 8%/year × 3 years.

July 18th, 2019, 12:02 AM  #8 
Senior Member Joined: Apr 2014 From: UK Posts: 953 Thanks: 340 
Yeah, but the real shortcut is not needing to know it's 8% at all, as post #6


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