The machine must have gone at the rate of
seven-twenty-fourths of a mile per minute and the wind travelled
five-twenty-fourths of a mile per minute.
Thus, going, the wind would
help, and the machine would do twelve-twenty-fourths, or half a mile a
minute, and returning only two-twenty-fourths, or one-twelfth of a mile
per minute, the wind being against it.
The machine without any wind
could therefore do the ten miles in thirty-four and two-sevenths
minutes, since it could do seven miles in twenty-four minutes.