They key to solving this problem is to realize that the speed of the
boat is REDUCED by 5 km/hr when it's traveling UPSTREAM against the
current. And it is INCREASED by 5 km/hr when it is traveling downstream
with the current.
Then, if we let S = the speed of the boat in still water (which is the same as saying ZERO current)
then S - 5 = the speed of the boat going upstream
and S+5 = the speed of the boat going downstream
The other thing we need to remember is that the TIME = Distance / Speed
So, the time to travel upstream is 195 / (S - 5)
and the time to travel downstream is 195 / (S+5)
Next, according to the problem, the difference between those two times is 50 hours.
So, 195 / (S-5) - 195 / (S+5) = 50
Now we can solve for S. To subtract fractions we need a COMMON DENOMINATOR