Problem: a sum of money at simple interest amounts to $ 680 in 3 years and to $ 710 in 4 years . the sum is :
Rationale: "c $ 590 s . i . for 1 year = $ ( 710 - 680 ) = $ 30 . s . i . for 3 years = $ ( 30 x 3 ) = $ 90 . principal = $ ( 680 - 90 ) = $ 590 ."
annotated_formula: subtract(680, divide(multiply(subtract(710, 680), 3), 4))