Let's define a binary strangle as an option that pays 1 if ST>=K2 or if ST<=K1. Here, we have K1=15 and K2=20. You have access to the following binary calls. What is the time-0 price of the binary strangle? Assume a discount factor of Z0=0.9. The binary calls are given in the format of (Strike K,Price C0)
(15,0.73)(17.5,0.34)(20,0.13)