24. $a, b, c, d$ are single-digit numbers, and $\overline{a b}-\overline{c d}=21, \overline{c d 1}-\overline{1 a b}=6$. Then $\overline{a d}$ equals what?

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Note: The notation $\overline{xy}$ represents a two-digit number with digits $x$ and $y$, and $\overline{xyz}$ represents a three-digit number with digits $x$, $y$, and $z$.
Reference answer: 34