Is it permissible to combine and shorten prayers in Paris for 15 days, even if one travels to places more than 80 km away from the place of residence in the same city? And what is the ruling on combining and shortening prayers before traveling by plane to avoid missing prayers?

A stay exceeding four days nullifies the concessions of travel, making the resident like a non-traveler. Therefore, he must complete the four-rak'ah prayers, and he is not permitted to combine prayers except for another valid excuse. As for moving around within the city, it does not permit availing oneself of concessions. However, it is permissible to avail oneself of concessions by shortening and combining prayers during travel, after departing from inhabited areas, and before entering the city of residence or the original homeland. It is not permissible to shorten and combine prayers within the city limits where a person resides, even if he is heading to the airport, because he is considered a resident.