Problem: the population of a town is 10000 . it decreases annually at the rate of 20 % p . a . what will be its population after 2 years ?
Rationale: "10000 × 80 / 100 × 80 / 100 = 6400 answer : a"
annotated_formula: subtract(subtract(10000, multiply(10000, divide(20, const_100))), multiply(subtract(10000, multiply(10000, divide(20, const_100))), divide(20, const_100)))