Jane took 30 min to drive her boat upstream to water-ski at her favorite spot. Coming back later in the day, at the same boat speed, took her 15 min. If the current in that part of the river is 3 km per hr, what was her boat speed in still water?
Distance = Rate x time
Going upriver D = (x-c) * 1/2 hr where x= boat speed c = current speed
Going DOWNriver D = (x+c) * 1/4 hr
The D's are the same distance...
(x-c)* 1/2 = (x+c) * 1/4
1/2 x - 1/2 c = 1/4 x +1/4c
1/4x = 3/4c
x = 3 c c= 3 km/hr) x = boat speed = 3 * 3km/hr = 9 km/hr in still water
(Does she TRY to ski behind THIS slow boat??)
+129840 Let the boat rate in still water [ in kph] = B and with the current = B + 3
Let the boat rate against the current = B - 3
And Distance / Rate = Time......but it takes twice as long against the current as with the current
So
2 [ D / (B + 3)] = D / ( B - 3) cancel the D's
2 / ( B + 3) = 1 / ( B - 3) cross-multiply
2 ( B - 3) = B + 3
2B - 6 = B + 3 rearrange
B = 9 kph
