Two trains

Two trains leave a station at the same time, heading in opposite directions. One train is traveling at 80 km-h, the other at 90 km-h. How long will it be for the trains to be 425 km Apart? 

distance = rate x time
d = rt

One train
rate = 80
time = t {they left at the same time}
d = 80t {distance = rate x time}

Other train
rate = 90
time = t {they left at the same time}
d = 90t {distance = rate x time}

To find out when they are 425 apart, add the distances and set equal to 425

80t + 90t = 425 {added distances and set equal to 425}
170t = 425 {combined like terms}
t = 2.5 {divided both sides by 170}

In 2.5 hours, they will be 425 km apart

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