a man can do a piece of work in 7 days , but with the help of his son , he can do it in 3 days . in what time can the son do it alone ?
"son ' s 1 day ' s work = ( 1 / 3 ) - ( 1 / 7 ) = 4 / 21 the son alone can do the work in 21 / 4 days answer is d"
divide(multiply(7, 3), subtract(7, 3))