Strange Math

Two problems that will make you want to throw a chair at someone, from John Jackson, Rational Amusements for Winter Evenings, 1821:

I.

If from six ye take nine, and from nine ye take ten
(Ye youths, now the mystery explain;)
And if fifty from forty be taken, there then,
Shall just half a dozen remain.

II.

One third of twelve, if you divide,
By just one fifth of seven,
The true result (it has been tried,)
Exactly is eleven.

How? Why?

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