So,
All we have to do is multiply the amount of water Andy carried by the fractional amount he drank.
1/2 * 2/3
This is pretty simple. To halve TWO-thirds, just halve the numerator, since it's such a nice number.
Presto. Half of two is one.
Therefore,
1/2 * 2/3 = 1/3
Andy drank 1/3 gallons of water on his hike. That's 5 1/3 cups of water. I would estimate his hike to be about 3 miles. Easy peasy.