Due to increase the rate of simple interest from 5% to 7%, the income of Rahim was increased by Tk. 1200 in 3 years. How much was his principal?

Question: Due to increase the rate of simple interest from 5% to 7%, the income of Rahim was increased by Tk. 1200 in 3 years. How much was his principal?

Solution:
Let,
His principal = P

ATQ,
{P × (7/100) × 3} - {P × (5/100) × 3} = 1200
⇒ (21P/100) - (15P/100) = 1200
⇒ (21P - 15P)/100 = 1200
⇒ 6P/100 = 1200
⇒ 6P = 1200 × 100
⇒ P = 120000/6
∴ P = 20000