Explanation:

Take two variables, one for the speed of boat in still water (x) and other for the speed of stream (y).

Therefore, with speed = distance / time,

1.) x+y=30/2, x+y=15 ( for downstream)

2.) x-y=30/6, x-y=5 (for upstream)

adding (1.) to (2.)…

2 x =20, x=10

Hence, option 4 is the correct answer.