a sum of money at simple interest amounts to 815 in 3 years and to 854 in 4 years . the sum is :
"a 698 s . i . for 1 year = ( 854 - 815 ) = 39 . s . i . for 3 years = ( 39 x 3 ) = 117 . principal = ( 815 - 117 ) = 698 ."
subtract(815, divide(multiply(subtract(854, 815), 3), 4))