Problem Archive

Two food trucks are catering at a local festival.
Truck A can serve a full lunch to 160 people in 4 hours.
Truck B can serve the same number of people in 2.5 hours.
If they work together (at full speed), how long will it take them to serve 160 people?

Stretch my thinking...
What if there were 200 people? What about 500?

Four people need to cross a narrow, rickety bridge at night. They have one torch, and the bridge is too dangerous to cross without it. At most, two people can cross at a time, and they must move at the slower person's pace when crossing together.

Each person takes a different amount of time to cross:

Person A: 1 minute
Person B: 2 minutes
Person C: 5 minutes
Person D: 10 minutes

What is the fastest time for all four people to get across the bridge?

Constraints:
The torch must be carried on each crossing.
No more than two people can be on the bridge at a time.
When two people cross together, they move at the pace of the slower person.
Get everyone across the bridge in the least amount of time possible.