if a is thrice as fast as b and together can do a work in 21 days . in how many days a alone can do the work ?
"a ’ s one day ’ s work = 1 / x b ’ s one day ’ s work = 1 / 3 x a + b ’ s one day ’ s work = 1 / x + 1 / 3 x = 1 / 21 = 3 + 1 / 3 x = 4 / 3 x = 1 / 21 x = 21 * 4 / 3 = 28 answer : c"
inverse(divide(const_3, multiply(21, const_4)))