I am self-studying for an actuarial exam on financial economics. I encountered this problem, and I am having difficulty seeing why the statement underlined is true:
How do we know that $P(60) - C(60) + P(50) - C(50) = 110e^{-rT} - 2Se^{-\delta T} = (2/3)(15)$?
The part with (2/3) confuses me. We are using 2 out of the 3 equations, but I don't see why that would necessarily imply that the value of adding 2 of the 3 equations is (2/3) the value of adding the three equations together.