Answer
1) Andrew and Blake will cross the shaft; Andrew will come back
2) Johnson and Kelly will go; Blake will come back
3) Andrew and Blake will cross the shaft
Work Step by Step
Andrew and Blake will cross the shaft in 2 minutes, Andrew will come back in 1 minute, Johnson and Kelly will cross in 8 minutes, Blake will come back in 2 minutes and Andrew and Blake will again cross the shaft in 2 minutes,
total time $= 2+1+8+2+2 =15$
Hence they crossed in 15 minutes.
The main step in realizing the solution was that the person coming back does not need to be from the persons who just crossed the shaft.
