Question 971842
The ques. is wrong. In same amount of time, distance travelled upstream should be less than distance travelled downstream. 
So, Solving the ques, by reversing the data as upstream distance=29 miles and downstream distance =58 miles

Let boat's speed in still water = b,
Since time taken upstream = Time taken downstream
=> Distance upstream/ Net speed upstream = Distance downstream/net speed downstream
=> {{{29/(b-4) = 58/(b+4)}}}
=> {{{b+4 = 2b-8}}}
=> {{{b=12}}}
So speed of boat in still water = 12 mph.