An electrician can wire a house in 20 hours. if she works with an apprentice, the same job can be completed in 12 hours. how long would it take the apprentice, working alone, to wire the house?

We know that
An electrician can wire a house in > 20 hours
so
100%> 20 hours
 X> 12 hour
x=12*100/20> x=60%

that means 
the apprentice in 12 hours complete the (100-60)> 40%

the apprentice in 12 hours complete > 40%
x hours> 100%
x=100*12/40> x=30 hours

the answer is
30 hours