Cruise liner

The cruise liner runs along the river between points A and B. The journey downstream takes 40 minutes and 1 hour upriver. The flow rate of the river is 3 km/h. What is the speed of a cruise liner?

Correct answer:

v =  15 km/h

Step-by-step explanation:

$$\begin{gathered} t_1=0:40=0hr40min=0+\frac{40}{60}=0.6667hr\doteq 0.6667 \\ {t}_2=1\text{h} \\ (v+3)\cdot t_1=(v-3)\cdot t_2 \\ (v+3)\cdot 0.66666666666667=(v-3)\cdot 1 \\ 0.333333v=5 \\ v=\frac{5}{0.33333333}=15 \\ v=15=15\text{km/h} \end{gathered}$$

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