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18. If a sum of money amounts to Rs. 2175 in 3 years and to Rs. 2625 in 5 years, find the rate of interest.

a. [tex]$15 \%$[/tex]
b. [tex]$12 \%$[/tex]

Sagot :

To find the rate of interest for the given problem, we can follow these steps:

1. Identify the given values:
- Initial principal (after 3 years): Rs. 2175
- Final principal (after 5 years): Rs. 2625
- Time period from initial to final amount: 5 years - 3 years = 2 years

2. Calculate the interest accumulated over the 2 years:
- Interest = Final principal - Initial principal
- Interest = Rs. 2625 - Rs. 2175 = Rs. 450

3. Determine the principal amount initially (at the start of the 2-year period):
- Principal_initial = Rs. 2175

4. Calculate the rate of interest per annum:
- We know that the interest accumulated over 2 years is Rs. 450.
- Using the simple interest formula [tex]\( SI = P \times R \times T / 100 \)[/tex], we can find the annual rate of interest [tex]\( R \)[/tex].
- Rearrange the formula to solve for [tex]\( R \)[/tex]: [tex]\( R = (SI \times 100) / (P \times T) \)[/tex]
- Substitute the values into the formula:
[tex]\[ R = \left(\frac{450 \times 100}{2175 \times 2}\right) \][/tex]
- Simplify:
[tex]\[ R = \left(\frac{45000}{4350}\right) = \left(\frac{100}{9.66666667}\right) \approx 10.344827586206897 \][/tex]
- Therefore, the rate of interest per annum is approximately 10.34%.

In summary, the rate of interest for the given sum of money is approximately 10.34% per annum.