The simple interest on a certain sum at the rate of 5% per annum for 3 years and 7 years differs by Tk. 400. What is the principal amount?

Question: The simple interest on a certain sum at the rate of 5% per annum for 3 years and 7 years differs by Tk. 400. What is the principal amount?

Solution:
Let the principal amount be Tk. P
Then, ATQ
400 = {(P × 5 × 7)​/100} - {(P × 5 × 3​)/100}
⇒ 400 = (35P - 15P)/100
⇒ 40000 = 20P
⇒ P = 40000/20
∴ P = 2000

So the principal amount is Tk. 2000