Workings

Hint
You can either fix the distance rowed and see what numbers work, or you can fix the number of hours.

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.

There are a number of ways of working this out, and here are two of them:

Method 1
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
 2   4

Multiplying throughout by 4 gives:

 2D − D = 12

So:

 D = 12 miles

They rowed 12 miles upstream.

Method 2
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

Giving:

 2H = 4H − 12

Rearranging gives:

 12 = 2H

So:

 H = 6 hours

They rowed for 6 hours upstream at 2 miles per hour, which is a total of 12 miles.