Four people are trying to get across a bridge the zombies are coming so they only have 15 minutes. person 1 takes 1 minute to get across person 2 takes 2 minutes person 3 takes 5 and person 4 takes 8 minutes

two may cross at once and they will go at the same pace as the slowest on the bridge and they need a torch to cross

firstly a and b will cross elapsing a total time of 2 minutes  a because he is the fastest will bring the torch back which takes it up to three minutes he passes the torch to c and d who cross in 8 minutes bringing the time up to 11 minutes then b will come back with the torch taking 2 minutes taking it up to 13 minutes and bring over a taking a total of 15 minutes.

yay

or alternatively a and b could leave behind c and d the slow people they are.

they may not be fast but they make good zombie bait!

and Christmas Pikachu