If time upstream = n × time downstream and speed in still water is 'x' and speed of stream is 'y', then find x :y.

Time upstream = n× Time downstream

Speed in still water = x

Speed of the stream = y

​$$\frac{n}{1} = \frac{x + y}{x - y}$$ 

$$\text{Time} = \frac{ \text{Distance}}{\text{Speed}}$$  ​

Let the distance traveled upstream and downstream be the same, denoted by D.

Speed upstream = (x - y) and Speed downstream = (x + y)
According to the question, time upstream = n × time downstream:
Time = Distance / Speed
​$$\frac{D}{(x - y)} = n \times \left(\frac{D}{(x + y)}\right) \\\ \\\frac{1}{(x - y)} = \frac{n}{(x + y)} \\\ \\(x + y) = n \times (x - y) \\\ \\x + y = nx - ny \\\ \\x - nx = -y - ny \\\ \\x(1 - n) = -y(1 + n) \\\ \\\frac{x}{y} = \frac{(n+1)}{(n-1)}$$​​
Thus, the ratio of x:y is (n +1):(n -1).