Let's imagine the yard contains 90 items to be cleared. Since Joe takes 45 minutes to remove the 90 items to clear the yard, he's working at the rate of 1/2 (45/90) minute to remove one item, or 2 items (90/45) every minute. Jan would remove 90 items in 30 minutes, so that's 3 items (90/30) a minute or one item in 1/3 minute (30/90).
If they work together, in one minute they could remove 5 items (2+3). To remove 90 items it would take 90/5=18 times as long=18 minutes to do the job.