It takes one person 12 minutes to do a job, another perspn 18 minutes. How long would it take for both of them to do the job together?

I figured 12 minutes is 1/5 of an hour(60 minutes) and 18 minutes is 3/10 of an hour. 1/5 = 2/10. 2x/10 + 3x/10 =10/10 or 1. 10/1 X 5x/10 =5x. 10 X 10 =100. 5x = 100. x= 20 minutes, but the answer in the back of the book was 7 1/5 minutes. How did I figure it wrong? I know that with 2 people wprking it should take less time.