walking 5 / 4 of his usual rate , a boy reaches his school 4 min early . find his usual time to reach the school ?
"speed ratio = 1 : 5 / 4 = 4 : 5 time ratio = 5 : 4 1 - - - - - - - - 5 4 - - - - - - - - - ? 20 m . answer : e"
multiply(4, 5)