First, John takes the flashlight and crosses the bridge with Paul, which takes 2 minutes.   John then returns with the flashlight, which takes 1 more minute (3 minutes have already passed).  John gives the flashlight to George. Both George and Ringo cross the bridge together, which takes 1 minute (13 minutes have passed so far).  George then gives the flashlight to Paul, who comes back for John taking two more minutes (15 minutes)  Now, John and Paul cross the bridge again, taking 2 minutes. Hence, they reach the concert in 17 minutes.