Answer: The trick to this question is twofold:
Spot that 94 is double 47. Therefore when going downstream, the speed of the boat relative to the earth is double it's speed going upstream as it covers double the distance in the same time.
Understand that whatever the speed of the boat is, when going downstream it's speed relative to the earth is 6mph greater than speed relative to the water, and conversely when going upstream, earth speed is 6mph less than water speed.
So, if the speed through the water is X, upstream speed is (X-6) and downstream speed is (X+6).
Now, we know that upstream speed is double downstream speed, so:
Double (x-6) is equal to (X+6)
Or:
2(x-6)=(X+6)
