Question 389207
It will take : 1/(1/12 + 1/15) = 3*4*5/(5+4) = 20/3 = 6 + 2/3 minutes or 6 minutes 40 seconds.
 
The reasoning can be done like this : the copy-speed of A is 300/15 pages/minutes
 
B is 300/12 pages/minute, if they work together, the total speed is 300/15+300/12=20+25=45 page minutes, so for the 300 pages, it will take : 300/45 = 6,666 minutes, or 6 2/3 minutes.