In order to find the unit rate of acres per day, or in other words, how many acres can they plant in one day, you must divide 5/8, the number of acres planted, by 4/5, the number of days it took to plant. When you divide the two fractions, first set up an expression:
5/8÷4/5
Then, to make things simpler, turn the division expression into a multiplication statement so that you have 5/8*5/4. Then multiply straight across and you get your answer as 25/32. This means that the workers can plant 25/32 acres per day.