Given: Simple interest for 9 years = (3/5) × Principal Concept used: Simple interest, SI = PRT/100 Calculation: P × R × 9/100 = (3/5) × P R = (3/5) × (100/9) = 20/3% Alternate Method: Let the principal be 5x. ⇒ Simple interest = 3x ⇒ Simple interest =
PRT
100
⇒ 3x=
5x×R×9
100
⇒ R = 300/45 = 20/3 % ∴ The rate of interest per annum is 20/3%.