a man can do a piece of work in 5 days , but with the help of his son , he can do it in 4 days . in what time can the son do it alone ?
"son ' s 1 day ' s work = ( 1 / 4 ) - ( 1 / 5 ) = 1 / 20 the son alone can do the work in 20 days answer is c"
divide(multiply(5, 4), subtract(5, 4))