Question 362901
There is no importance of the average speed of boat as it's useless variable to calculate the current speed of water..If the boat can go 33 and 21 miles in same amount of time we could think such that;

Assume "x" as boat speed without effect of current and "y" as the current speed..

x + y = 33

x - y = 21

Sum up the equations therefore >> 2x = 54 and x = 27 >> 

Put "x" into any equation you'll get >> y = 6 miles/h

RF.