What's the quickest amount of time that all four kids can get across a bridge with on kid always going back to get the other.
1 kid took 5 minutes
1 kid took 10 minutes
1 kid took 20 minutes
1 kid took 25 minutes
there's a lot unsaid here.
Do all the kids start walking at the same time, so each return trip is shorter?
Does the returning kid carry the other one, or do they always have to walk at the slower kid;s speed?
Assuming the fastest kid always has to walk all the way back to the start, and walk at the slower kid's speed, then
kids 1&2 walk across in 10 minutes.
kid1 walks back in 5
kids 1&3 walk across in 20
kid1 walks back in 5
kids 1&4 cross in 25
total time: 65 minutes