carl can wash all the windows of his house in 6 hours . his wife maggie can wash all the windows in 7 hours . how many hours will it take for both of them working together to wash all the windows ?
"work hrs = ab / ( a + b ) = 42 / 13 = 42 3 / 3 answer is c"
multiply(divide(const_60, add(divide(const_1, 6), divide(const_1, 7))), add(divide(const_1, 6), divide(const_1, 7)))