Assume that the water inflow per minute is a and the water inflow per minute is b, and everyone scoops water once a minute.

36c=3a+b

50c= 10a+b

So 14c=7a.

a=2c

So b=30c.

Suppose you need to line up x people.

2cx=2a+b=4c+30c=34c

So x= 17.