Solve a Simple Physics Problem: Boat Speed and River Flow in 30 Minutes

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Homework Statement


A boat can move at 30 km/hr in still water. how long will it take to move 12 km upstream in a river flowing 6.0 km/hr.

I know the answer is 30 min but the way my teacher explained it is very difficult to remember can someone show me another way to do this problem?

the way she showed us:
Vbg = 30 km/hr
Vbr = Vbg + Vgr = 24 km/hr = 0.4 km/min
Vrg = 6.0 km/hr
Vgr = -6.0 km/hr + 12/0.4 = 30 min
 
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Discussion

Do you agree that adding the velocity of the boat relative to the water + velocity of the water = velocity of the boat relative to ground (ie: net velocity) ?