Answer:
The speed of the stream is 22.36 km/h.
Step-by-step explanation:
The course of the boat contains two stages of equal length. On the first one he is going downstream, so its resultant speed is "30 + x" km/h, where "x" is the speed of the stream. On the other hand for the second part of the course he is going against the stream, so the resultant speed is " 30 - x" km/h. The time of each course must be:
[tex]t_1 = \frac{30}{30 + x}[/tex]
[tex]t_2 = \frac{30}{30 - x}[/tex]
The sum of these times must be equal to the total time of the course, therefore:
[tex]t_1 + t_2 = 4.5\\\frac{30}{30 + x} + \frac{30}{30 - x} = 4.5\\\frac{30*(30 - x) + 30*(30 + x)}{(30 + x)(30 - x)} = 4.5\\900 - 30*x + 900 + 30*x = 4.5*(900 - x^2)\\4050 - 4.5*x^2 = 1800\\4.5*x^2 = 4050 - 1800\\4.5*x^2 = 2250\\x^2 = \frac{2250}{4.5}\\x^2 = 500\\x = 22.36[/tex]
The speed of the stream is 22.36 km/h.
