carl can wash all the windows of his house in 7 hours . his wife maggie can wash all the windows in 4 hours . how many hours will it take for both of them working together to wash all the windows ?
work hrs = ab / ( a + b ) = 28 / 11 = 11 6 / 2 answer is c
multiply(divide(const_60, add(divide(const_1, 7), divide(const_1, 4))), add(divide(const_1, 7), divide(const_1, 4)))