Note on elasticity problem parts a and b: you can do the midpoint formula but they will also accept you computing the percent change in quantity over the percent change in price. For example, in part a, when the quantity went from 100 down to 30 that is a 70% drop (70/100) and when the price went up from $1.00 to $1.35 that is a 35% increase (.35/1.00). So this is 70/35 and the answer is 2. These numbers were not friendly for the midpoint formula without a calculator. If you did the midpoint formula your answer will be different but they will still accept it. For part b The quantity supplied didn’t change so that’s a 0% change in quantity divided by a 35% change in price so the elasticity of supply is 0.