Can you solve the bridge riddle?

Contents

Problem

A person Mr. P has taken a remote job in a hilly area. Out of curiosity, he pulls the skull symbolized lever. Doing that releases the mutant zombies very fast. The person is not alone, he has the janitor, the lab assistant and the old professor with him. To escape and save all, there is only one route i.e., the old rope bridge, spanning a massive gorge.

Now Mr. P takes 1 minute to cross the bridge. The lab assistant, janitor and the old professor takes 2, 5 and 10mins to cross. The bridge can hold only two people at a time. Moreover, the tiny old lantern is the only light source. The two people crossing the bridge, must have the lantern with them to cross. The zombies are supposed to catch up to them in 17mins. Therefore, in what way they should cross the bridge so that everyone survives?

Remember, you can neither swing across the bridge, nor can you use the bridge as a raft.

Solution

Really, it can be solved easily with a simple idea. To minimize the wastage of time, two slowest people must cross together. But we also need to keep in mind that we need someone fast to return the lamp.

Therefore, the first crossing must be done by Mr. P and the lab assistant. This will take 2 minutes. Then Mr. P would go back to the other side with the lantern. That will take another 1min.

Now Mr. P would pass on the lantern to the janitor and professor. They will then cross the bridge together. This would take 10minutes in total.

Now they would pass the lantern to the lab assistant who was already waiting for them on the other side.

She would then run back the bridge with the lantern to get Mr. P. This would consume 2more minutes.

And at last Mr. and the Lab assistant would cross the bridge (taking 2mins again), come on the other side and cut the ropes of the bridge.

Thus, Mr. P crosses the bridge in time and saves himself and everyone else.