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It would take B 21/4 hours to reach point Q.
During this time A walked 3*21/4 and the difference between the point where he is and Q at the moment A reached Q is 21/4 miles.
Now they are walking toward each other: t - time they spend walking
t*(3+4)=21/4 => t=3/4. B would walk 4*3/4=3miles.
Then the distance is 21-3=18 miles.
The distance between P and R = PT + TR
B reaches Q in 21/4=5,25 hours. In 5.25 hours A passes 3*5.25=15.75 km PT=15.75
Then, B starts back, while A is still on its way to Q
The distance between A and B Is 5.25, the speeds are 3 and 4, so we have to do next is to find when they will meet:
5.25/(3+4)=0.75 hour. In 0.75 hour A will pass additional 0.75*3=2.25 km TR=2.25
So the answer is PT+TR=15.75+2.25=18km