working alone , a can complete a certain kind of job in 12 hours . a and d , working together at their respective rates , can complete one of these jobs in 4 hours . in how many hours can d , working alone , complete one of these jobs ?
"let total time taken by s to complete the job = s total time taken by a to complete the job = 12 work done by a in an hour 1 / a = 1 / 12 working together a and d can complete the job in 4 hours 1 / a + 1 / d = 1 / 4 = > 1 / d = 1 / 4 - 1 / 12 = 1 / 4 - 1 / 12 = 1 / 6 = > d = 6 hours answer d"
inverse(subtract(inverse(4), inverse(12)))