Let the speed of the boat be x kmph and stream be y kmph
Downstream = x + y kmph and Upstream = x - y kmph
Given that 24 / (x-y) + 28 / (x+y) = 6 and
30 / (x-y) + 21 / (x+y) = 13 / 2
Upon solving we get x= 10 and y = 4
Speed of the stream = 4 km/h

Let the speed of the stream be x km/hr
Given, speed of boat = 15 km/hr

Then,
Speed downstream = (15 + x) km/hr,

Speed upstream = (15 - x) km/hr.

Given, Distance covered by boat downstream =30
So,Distance covered by boat upstream =30

Given, Time for upstream and downstream = 4 hr 30 mins = 4 (1/2) hr
=> (Upstream Distance/ Speed upstream) + (Downstream Distance/ Speed downstream) = Total Time
=> [30 / (15 + x)] +[30 / (15 - x)] = 4 (1/2)
=> 30 [1/ (15 + x)] +[1/ (15 - x)] = 9 / 2
=> 30 [(15 - x) +(15 + x) / (15² - x²)] = 9 / 2
=> 30 [30 / (225 -x²)] = 9 / 2
=> 900 /(225 - x²) =9 / 2
=> 900 * (2 / 9) =(225 - x²)
=> 200 = 225 - x²
=> x² = 25
=> x = 5
Thus,speed of the stream = x = 5 km/hr

Given, speed of boat = 5 km/hr
Speed of the stream = 1 km/hr

Formula:

If the speed of a boat in still water is x km/hr and the speed of the stream is y km/hr, 
then:

Speed downstream = (x+y) km/hr.
Speed upstream = (x-y) km/hr.

=> Speed downstream = 5 + 1 = 6 km/hr
=> Speed upstream = 5 - 1 = 4 km/hr

Let the required distance be "x" km
Given , Time = 75 min = 75 / 60 hr

By formula: Time = Distance / Speed
=> (x/6) + (x/4) = 75 / 60
=> 10x / 24 = 75 / 60
=> x = (75 * 24) / (60 * 10)
=> x = 1800 / 600
=> x = 3 km

Distance = d
x = speed of the stream
Downstream = 6 + x
Upstream = 6 - x
Then,
d / (6+x) = d / (6-x)
=> 2 / (6+x) = 1 / (6-x)
=> 12 - 2x = 6 + x
=> 6 = 3x
x = 2 kmph

Speed of the boat = x and speed of the stream = y 
Upstream Speed = x - y
Downstream Speed = x + y
=> x / y = 15/5 = 3 kmph
Again x = 4y
x - y = 3
=> y = 1 and x= 4
Downstream Speed = x + y = 5 kmph
Time = 15 / 5 = 3 hr