Question 1186392
sadly this problem is full of misleading bs. all you need to know is at the start of the bridge a floating object is carried by the stream. in 20 mins it was carried for 1 mile. 

to get someplace in 20 mins instead of 60 mins (60/20=3) means you traveled 3 times faster.
so if 1mph is base speed, 3 times that is 3mph, this would be the current of the stream.