Flight Attendant
Just
a few minutes before takeoff, a flight attendant notices some
interference.
She then asks the passengers:
"Who has their computer on?" Fifteen hands go up.
"Who has their phone on?" Thirteen hands go up.
"Who has both on?" Seven people raise their hands.
"Who has neither a computer nor a phone on?" Nine people respond.
Question:
How many passengers are there on the plane? (Not counting the flight
crew)
Answer: 30
There are 30 passengers (excluding the crew, of course).
We know that 7 people have both a computer and a phone on.
We can deduce that: 8 people have only their computer on (15-7), 6 have only their phone on (13-7).
We also know that 9 people have no devices on.
7 + 8 + 6 + 9 = 30.
