Let u be the boat speed in still water ( in  ), and let v be the current speed ( in  ).
Then you have two equations


7*(u-v) = 140,    (1)    and
5*(u+v) = 140.    (2)

Indeed, u-v is the boat's speed relatively the river's bank when it is traveling upstream, and
u+ v is the boat's speed relatively the river's bank when it is traveling downstream.

From (1) and (2) you have 

u - v =  = 20,   (1')     and
u + v =  = 28.   (2')

Now add (1') and (2'). You will get 2u = 20 + 28 = 48. Hence, u =  = 24 .

Then from (1') v = u - 20 = 24 - 20 = 4 .

Answer. The speed of the boat in still water is 24 .