Loc can mow his entire lawn in 20 minutes; Loc's roommate Reza needs 30 minutes to mow the entire lawn. If Loc starts mowing at 1:00, mows

So, what is your question? If you want to know how long would it take them, working together, to mow the lawn, then it would be:

 

1/20 + 1/30 =1/12, or 12 minutes working together.