Answer:
The current is 5 mph
Explanation:
The current helps the boat going down stream and hinders going up so it adds to the speed downstream and subtracts going up. 
speed downstream is 20miles/40minutes =1 mi per 2 min. Or 30 mph 
Speed upstream is 
20 miles/ 60 minutes = 1 mile per 3minutes or 20 mph 
So if the speed of the boat is the same (s) then downstream the boat travels 
S+c =30 mph and s-c =20mph upstream if c is tge current speed 
So s=30-c and s=20+c 
So. 30-C=20+c 
 10=2C 
 5=C 
The current is 5 mph