Answer to Question 1
The midpoint formula is a convention of calculating elasticity percentages using the values halfway between the initial and new price and quantity demanded values. It is used so an elasticity calculation will not depend on the direction of the change in quantity demanded due to a change in price.
Answer to Question 2
If you put the money in your savings account, you will have 105 to spend next year on the item (or items) of your choice. Thus, the opportunity cost of spending the money today is the enjoyment you give up by not having the 105 to buy something next year.