Drawing a model is a great idea as it allows you to see things clearly. Initially you have 4units to 5units, or 4:5 for spoons and forks. After buying and selling, we know they are equal, hence 4u + 40 : 5u - 720 is 1:1. After cross multiplication (redundant because it's 1:1 but still a good habit to have), we get 5u - 720 = 4u + 40. After solving, you can get u=760 and solve for spoons = 4u + 40, which is 3080.