A sum of Rs. 10 is lend to be returned in 11 monthly instalments of Rs...
Given :
A sum of Rs. 10 is lend to be returned in 11 monthly instalments of Rs . 1 each interest being simple
Formula used :
Simple interest = P × R× T /100
Principal(P), Rate(R) and Time(T)
Calculation :
Let the rate of interest be R% per annum
⇒ Amount to be paid (if paid at the end of 11 months)
⇒ 10 + [10 × R × (11/12) / 100] = 10 + (11R/120)
⇒ Total effective payment = (Rs. 1 + interest on Rs. 1 for 10 months) + (Rs. 1 + interest on Rs. 1 for 9 months) + .... +(Rs. 1 + interest on Rs. 1 for 1 months) + Rs. 1
⇒ (1 + 1 × R × (10/12) / 100) + (1 + 1 × R × (9/12) / 100) + .... + (1 + 1 × R × (1/12) / 100) + 1
⇒ (1 + 10R/1200) + (1 + 9R/1200) + .... + (1 + R/1200) + 1
⇒ 11 + R(1 + 2 + .... + 10)/1200
⇒ 11 + R(10 × 11 / 2)/1200
⇒ 11 + R(10 × 11 / 2)/1200
⇒ 11 + 11R/240
Now we have,
⇒ 10 + 11R/120 = 11 + 11R/240
⇒ 11R/240 = 1
⇒ R = 240/11 = 21× 9/11%
∴ Rate of interest is 21(9/11)%