Cannot get the right answer

You can look at this as a rate problem. Jack takes 3 hours to cut one lawn and Marilyn takes 5 hours to cut one lawn. Another way of saying this is that Jack cuts 1/3 lawn per hour and Marilyn cuts 1/5 lawn per hour. If they work together you can combine those rates and get 1/3 + 1/5 = 5/15 + 3/15 = 8/15 lawn per hour together. 1 lawn divided by 8/15 lawns per hour is the same as 1 lawn times 15/8 hours/lawn =15/8 hours which is 1 7/8 hours total time if Jack and Marilyn work together.