A sum of money at simple interest amounts to ₹ 815 in 3 years and to ₹ 854 in 4 years. The sum is:
Simple interest for 1 year = ₹(854 - 815)
= ₹39
Simple interest for 3 years = ₹(39 x 3)
= ₹117
Principal = ₹(815 - 117) = ₹698.