Here it is

Question

An experienced electrician can wire a room twice as fast as an apprentice electrician. Working together, the electricians can wire a room in 3 hour. How long would it take the apprentice working alone to wire a room?

Answer 1

Love this kind of problems! :)
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Suppose that an experienced electrician can wire a room for x hours with productivity 1/x .
Then an apprentice electrician can wire a room for 2x hours with productivity 1/(2x) .
On the one hand their productivity is 1/3 , and on the other hand their productivity is (1/x + 1/(2x)) .
Now we are ready to write equation
1/x + 1/(2x) = 1/3   LCD(x, 2x, 3) = 6x
(6 + 3)/(6x) = 2x/(6x) ---> 9/(6x) = 2x/(6x) ---> 2x = 9 hours
                                                 

Answer 2

x/hr is speed of apprentice

2x/hr is speed of experienced electrician.

3hrs. *  ( 2x/hr + x/hr) = 1

3hrs. * [ (2x + x)/hr] = 1

3hrs. * ( 3x/hr) = 1

9x = 1

x = 1/9 room

 

so apprentice can wire a room alone in 9 hours.