Question 1148629
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Working together the whole time, the number of hours it would take them to do the job together is<br>
{{{(4*2)/(4+2) = 8/6 = 4/3}}}<br>
Bob takes 4 hours alone to clean the room.  He works 1 hour before Alan joins him, so he has done 1/4 of the job; 3/4 of the job remains to be done.<br>
It takes them 4/3 hours to do the whole job together; the number of hours it takes them to do the remaining 3/4 of the job is<br>
{{{(3/4)*(4/3) = 1}}}<br>
Since it was 4pm when they started working together, it will be 5pm when they finish.<br>