The trick is to get the two slowest people to cross the old cable bridge together

The key is to minimize the time wasted by the two slowest people by having them cross together.And because you'll need to make a couple of return trips with the lantern,you'll want to have the fastest people available to do so.So, you and the lab assistant quickly run across with the lantern,though you have to slow down a bit to match her pace.After two minutes, both of you are across,and you, as the quickest, run back with the lantern.