The swallows have gone

Dear friend, the swallows left this morning. The wires by the window, full all summer, are empty, and the house is quieter than I expected.

Dear friend, the swallows left this morning. The wires by the window are empty.

wire, swallows: $n=0$.

I did the sums: a swallow flies about sixty kilometres in an hour, and it flies for ten hours a day, so it covers six hundred kilometres a day.

A swallow flies about $60\mathrm{km}$ in an hour, for $10$ hours a day: $60\times 10=600\mathrm{km}$ a day.

$v=60km/h,t=10\mathrm{h}$ and $vt=600\mathrm{km}$ a day.

Their journey south is nine thousand kilometres. Nine thousand divided by six hundred is fifteen, so in fifteen days they will be in the warm again.

The journey south is $9000\mathrm{km}$, and $9000÷600=15$. In $15$ days they will be in the warm again.

$9000÷600=15$ days.

In spring they come back the same way. So every swallow flies eighteen thousand kilometres a year, twice the road, and still knows which wire is its own. Write to me when you see the first one.

In spring they come back the same way, so each year $2\times 9000=18000\mathrm{km}$, and each still finds its own wire. Write when you see the first one.

$2\times 9000=18000\mathrm{km}$ a year.