Question

# Using the ready reckoner, find the period of interest for an amount equal to Rs 16,939.2, when the principal is Rs 12000 at 9% p.a.

Hint: Find the compound interest using the amount and principal for R = 9%. Then find the compound interest for Re1. By using the ready reckoner table find the value corresponding to compound interest at 9%.

Ready Reckoner is a pre-calculated table of interest for different amounts and internal rates.
Acquired Interest per Rupee, compounded yearly,

 Years 9% 1 0.09 2 0.1881 3 0.2950 4 0.4116 5 0.5386

From the question, we have been given the Amount, principal and rate of interest.

Amount, A = 16939.2
Principal, P = Rs 12000
Rate of interest, r = 9%
First we need to find the compound interest.
Compound interest is the addition of interest to the principal sum of a loan or deposit, or in other words interest on interest.
$\Rightarrow$Compound interest = Amount – Principal = A – P
= 16939.2 – 12000 = 4939.2 Rupees.
Thus we have Rs. 4939.2 as compound interest for the principal of Rs. 12000 at 9%.
Now we need to find the compound interest for Re.1.
For Rs 12000, the compound interest is Rs 4939.2.
Therefore, for Principal Re.1, the compound interest $=\dfrac{4939.2}{12000}=0.4116$
Now from the ready reckoner, find the year corresponding to the compound interest 0.4116.
From the table you will get the value as 4.
Therefore, the period of interest, n = 4.

Note: The ready reckoner table is available for different rates of interest.
But for this problem on the value corresponding to the rate of interest 9% is required.
The ready reckoner table can be used to facilitate simple calculations, especially for applying the rates of discount, interest, charging etc.