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A certain delivery service offers both express and standard delivery

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A certain delivery service offers both express and standard delivery. Seventy-Five percent (75%)
of parcels are sent by standard delivery, 25% are sent by express. Of those sent standard, 80%
arrive the next day, and of those sent express, 95% arrive the next day.
a) What is the probability that any given parcel arrives next day?
b) Given that a parcel arrives the next day, what is the probability that it was sent express?