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July 16th, 2019, 07:47 AM   #1
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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?
Ganesh Ujwal is offline  
 
July 16th, 2019, 09:38 AM   #2
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$\displaystyle 1 + (0.08 )(3) = 1.24 = 124/100 = 62/50 = 31/25$
$\displaystyle (31/25)(Rs 800) = Rs 992$
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July 16th, 2019, 10:04 AM   #3
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How you got $\displaystyle 1 + (0.0 8 )3$?
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July 16th, 2019, 11:03 AM   #4
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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, $\displaystyle 100 \% + (\frac{8 \%}{year}) \cdot (3~years) = 124 \%$
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July 16th, 2019, 07:18 PM   #5
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Quote:
Originally Posted by DarnItJimImAnEngineer View Post
Rs 120 in...... by 1.24.
This part is OK and understood.


Quote:
Originally Posted by DarnItJimImAnEngineer View Post
If you want, $\displaystyle 100 \% + (\frac{8 \%}{year}) \cdot (3~years) = 124 \%$
How you wrote $\displaystyle 100 \% + (\frac{8 \%}{year}) \cdot (3~years)$?

Actual formula for $\displaystyle Amount = SI + P$.

Last edited by Ganesh Ujwal; July 16th, 2019 at 07:28 PM.
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July 17th, 2019, 12:02 AM   #6
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I did it this way:
920 + 0.03*3*800 = 992
weirddave is offline  
July 17th, 2019, 12:22 PM   #7
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Amount = SI + P implies Amount/P = 1 + SI / P = 100% + 8%/year × 3 years.
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July 18th, 2019, 12:02 AM   #8
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Yeah, but the real shortcut is not needing to know it's 8% at all, as post #6
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