Henry Ernest Dudeney/Modern Puzzles/59 - The Two Digits/Solution

by : $59$

 * The Two Digits

Solution
Examples given by :

It is worth pointing out that the second inadequate example quoted by himself:
 * $81 = \paren {1 + 8}^2$

which is ineligible because of the $2$, leads us on neatly into:
 * $\sqrt {81} = 1 + 8$

which unaccountably missed.

There may be more.

offers up:
 * $94 \equiv 4 \pmod 9$

which may or may not be eligible.

And indeed:

and so on.