Walkie-talkie on a 15 mile radius

Dennis and Chris Reed leave simultaneously from the same point hiking in opposite​ directions, Dennis is walking at 4 miles per hour and Chris at 2 mph. How long can they talk on their​ walkie-talkies if the​ walkie-talkies have a 15 mile​ radius?

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​Distance = rate x time
d = rt 

Dennis 
rate = 4
time = t    {left at same time}
d = 4t    {distance = rate x time} 

Chris
rate = 2
time = t    {left at same time}
d = 2t    {distance = rate x time} 

Combined distances must be no more than 15 miles. 

4t + 2t = 15    {added distances and set equal to 15}
6t = 15    {combined like terms}
t = 2.5 hours    {divided each side by 6} 

0.5 hours is 30 minutes. 

They can talk for 2 hours and 30 minutes,
or 2.5 hours as requested for rounding.

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