A sum of Tk. 10 is given as a loan, to be returned in six monthly installments of Tk. 3.What is the simple interest rate?

Let the rate of interest be R%
Amount due in 6 months
= 10 + simple interest on Tk. 10 for six months.
= {10 + 10 × R × (1/2)}/100
= 10 + (R/20)
With the formula mentioned,
3 = 100(10 + 9R/20)/{(100 × 6) + R × 6(6 - 1)}/(2 × 12)
⇒ 3 = (1000 + 5R)/{600 + (5R/4)}
⇒ 1800 + 15R/4 = 1000 + 5R
⇒ 5R/4 = 800
⇒ R = 640.
Hence interest rate is 640%