https://math.stackexchange.com/questions/5036578/what-is-the-refundable-option-on-expediacustomer-service

https://math.stackexchange.com/questions/5036578/what-is-the-refundable-option-on-expedia%ef%b8%8fcustomer-service

https://math.stackexchange.com/questions/5036578/what-is-the-refundable-option-on-expedia%ef%b8%8fcustomer-service