Strategy 1 involves using the person with the shortest crossing time (Rachel with 1 minute) to accompany each person across the bridge so the solo return time will always be the shortest possible.
This means the total time taken will be the sum of each person's crossing time (as they have a crossing time higher than that of Rachel's) + the 2 return journey's that Rachel must take to collect the next person.
It will be 2+7+10+1+1 = 21 minutes total time taken for everyone to be together on the other side of the bridge.
Strategy 2 takes a different journey where Rachel initially takes Ben across the bridge resulting in 2 minutes so far (as Ben's crossing time is 2 mins). She then crosses back again alone adding on another minute to the total time taken making it 3 minutes thus far. Instead of crossing back with another member this time Rachel stays on the left side of the bridge and allows George and Yvonne to cross and since Yvonne has the highest crossing time of 10 minutes, 10 minutes is added on to the total crossing time now making it 13 minutes. Ben then crosses back with the torch and collects Rachel to travel back completing the journey so that everyone has crossed the bridge and is together adding on another 2 minutes per trip across and back. Therefore final time taken for everyone to cross the bridge ultimately amounts to 13 + 4 = 17 minutes which is the fastest possible time. This is faster than strategy 1 because George's crossing time is taken out since he crosses with Yvonne who has a higher crossing time of 10 minutes. (However 3 minutes is still added on from R