One day, while walking through the woods, careless Humphry fell down a disused well. The well was 10 metres deep and the sides were green with slime.

 

Humphry was a good climber, but the sides were slippery and it took him 1 hour to climb up 1 metre. After an hour, Humphry lost concentration and slipped down ½ metre before gripping the wall of the well again.

 

Humphry continued to climb and slip this way until finally he managed to climb out of the well.How long did it take Humphry to climb back out of the well?