machine a can finish a job in 4 hours , machine в can finish the job in 2 hours , and machine с can finish the job in 6 hours . how many hours will it take for a , b , and с together to finish the job ?
"the combined rate is 1 / 4 + 1 / 2 + 1 / 6 = 11 / 12 of the job per hour . the time to complete the job is 12 / 11 hours . the answer is e ."
inverse(add(add(inverse(4), inverse(2)), inverse(6)))