Divide 500 by 4 to find out what a quarter of 500 is. When they sold 3/4 of the muffins, they had 1/4 left. 1/4 of 500 is 125.
Now we need to find out what 3/5 of 125 is. Divide 125 by 5, and you get 25. so mulitply that by 3 and you get your 3/5 = 75.
So the bakers had 75 muffins left.