Afarm can be used to generate electricity
a.solar
b.geothermald.
c.wind
d.hydroelectric

A wind farm can be used to generate electricity

      

Work backwards. double the percentage until it reaches 100%. how many times did you have to double it? divide 40 by the number of times you had to double it.
like so:
6.> 12.> > > 100
so you doubled it 4 times, then divide 40/4= 10 hours. so half life is 10 hrs.