The Bridge and Zombies Puzzle

One dark night, Layla and her brother Sami woke up to screams and the smell of smoke coming from everywhere. A disease had swept through the village and turned everyone into zombies. The family ran out of the village with the zombies right behind them. When they reached the bridge, Layla did some quick maths and worked out they had only 17 minutes for everyone to cross and then cut the bridge before the zombies caught up.

The problem: the bridge can't hold more than two people at once, and whoever crosses must carry the torch because it's pitch dark, and the family has only one. Sami needs 1 minute to cross, Layla 2 minutes, Mum 5 and Grandpa 10. When two people cross together, they move at the slower one's pace.

No tricks and no wordplay: there's a real logical solution. Pause the video, think it through, then check whether you got it right!