Let 4m be the principal. Then SI = (7/4) × 4m – 4m = 7m – 4m = 3m. Formula for simple interest SI = (P × R × T)/100 Where P is the principal R is the rate of interest T is the time ⇒ 3m = (4m × R × 4)/100 ⇒ 300m = 16m × R ⇒ R = 300m/16m ⇒ R = 18.75 %