A man can row 50 km upstream and 72 km downstream in 9 hours. He can

I got the answer in less than 30 secs without writing down a word. Since you are looking for faster approaches, let me write down my thought:

First thing that came to mind was 50/U + 72/D = 9
where U is the upstream speed, D is the downstream speed and D is obviously greater than U.
Now I thought of splitting 9 hrs into 5 hrs and 4 hrs. Why? There was a 50. 5 hrs was screaming out at me and then the remaining would be 4 hrs and 72 is divisible by 4, I know.
If I say that perhaps the time taken to go upstream was 5 hrs, the upstream speed, U, will be 10 km/hr.
If time taken to go downstream is 4 hrs, the downstream speed will be 72/4 = 18 km/hr
Now I need to check if U is actually 10 and D is actually 18. How?
I put it in the other equation.
70/U + 90/D = 12
70/10 + 90/18 = 12
7 + 5 = 12
These values of U and D satisfy.

So difference of speeds of boat and stream is 10 and sum of the speeds is 18.
This would give us two equations: B - S = 10 and B + S = 18
2B = 28
B = 14
So S = 4

GMAT does not require long calculations. If you find yourself making and solving tedious equations, be rest assured that the numbers will fall into place. The 9 hrs given above will not split into 5.2 and 3.8 hrs. It will split into 5 and 4 or 6 and 3 etc. You can pretty much guess the correct answer in the first go by looking at the numerator. 50 is not divisible by 6 but it is divisible by 5. And mind you, all this can be easily done orally. Try to minimize your use of pen and paper; your speed will automatically increase.