You are able to swim downstream at 3 miles an hour, &
upstream at 2 miles an hour. There is a difference of 1 mile an hour, which is
the river helping you in 1 direction, & slowing you in the other direction.
Average the 2 rates, & you have the rate that you can swim in still water,
which is 2.5 miles an hour. You can thus swim a mile in still water in 24