If a sum of Tk. 9 is lent to be paid back in 10 equal monthly installments of Tk. 1 each, the rate of interest is -

Let the rate of interest be R%
Amount due in 10 months
= 9 + simple interest on Tk. 9 for ten months
= 9 + {9 × R × (10/12)}/100
= 9 + (3R/40)
With the formula mentioned,
1 = 100{9 + 3R/40)}/[(100 × 10) + {R × 10(10 - 1)/(2 × 21)}]
900 + (15R/2) = 1000 + (15R/4)
15R/4 = 100
R = 26.67.
Hence the interest rate is 26.67%.