When the price of a good that complements a good decrease, then the quantity demanded of one increases and the demand for the other increases. When the price of a substitute good decreases, the quantity demanded that good increases, but the demand for the good that it is being substituted for decreases.
The chef will be able to prepare 3 servings of pasta because if you take one away from a whole number you have 9/10ths so you do that process again you have 2 servings but, you have 2 tenths left over so you add that to the 7 tenths.
