When is it hard to show if a set is closed or not closed?

Passengers usually start to board their flights 1 hour before the scheduled take off time. the gate will be closed 15 minutes before the takeoff time. jack’s plane is scheduled to take off at 12: 05 a.m. if he arrives at the gate at 11: 45 p.m., can he board the flight?

How many different three-digit numbers can be formed using the digits 1 comma 2 comma 9 comma 6 comma 4 comma 3 comma and 8 without repetition? for example, 664 is not allowed.

Afootball quarterback has two more chances to throw a touchdown before his team is forced to punt the ball. he misses the receiver on the first throw 25% of the time. when his first throw is incomplete, he misses the receiver on the second throw 15% of the time. what is the probability of not throwing the ball to a receiver on either throw?