Math Destroyer Help

[Fancy312]

Hi Guys!

Working on a math problem, and I need some help on the explanation of this one::: please and thank you to whoever can explain this

A boat travels between two riverside cities, A and B, located 21 miles apart. The river flows in the direction from A to B at 2 mph. The boat takes a total of 10 hours to make a round trip from A to B and back to A again. How long does it take the boat to make the one-way trip from A to B if the boats speed remailer constant relative to the speed of the river in each direction?

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[alexis2010]

Since the river flows in the direction from A to B , travel time should be less than 5 hours. Cross out C and D. Since the distance between A anb B is 21 miles, choice B looks good. Why? Because 21 divides 3 and 7 nicely. ( 7 is the time to go from B to A) or you can plug in the answers and see which one works .

 

[Fancy312]

Thank you so much @alexis2010