a sum of money at simple interest amounts to $ 780 in 3 years and to $ 930 in 4 years . the sum is :
"c $ 330 s . i . for 1 year = $ ( 930 - 780 ) = $ 150 . s . i . for 3 years = $ ( 150 x 3 ) = $ 450 . principal = $ ( 780 - 450 ) = $ 330 ."
subtract(780, divide(multiply(subtract(930, 780), const_2.0), 4))