Answer: 12 miles.
In the morning, rowing at 2 miles per hour, they rowed for 6 hours. In the afternoon, rowing at 4 miles per hour, they rowed for 3 hours. QED.
There are a number of ways of working this out, and here are two of them:
If we assume the distance the rowed upstream to be D miles, we know the morning took (D ÷ 2) hours, and the afternoon took (D ÷ 4) hours, with a difference of 3 hours. So:
D D 3
- - - =
Multiplying throughout by 2, and then 4, gives:
4D - 2D = 24
2D = 24
D = 12 miles.
They rowed 12 miles upstream. QED.
If we assume they rowed for H hours upstream, we know they travelled H x 2 miles in the morning. In the afternoon they rowed for (H - 3) hours, and travelled (H - 3) x 4 miles. We know these distances are the same, so:
2H = (H - 3) x 4
2H = 4H - 12
12 = 2H
H = 6 hours.
They rowed for 6 hours upstream at 2 miles per hour, which is a total of 12 miles. QED.