GS. 4 If the product of a 4-digit number abSd and 9 is equal to another 4-digit number dSba, find the value of $S$.
$a=1, d=9$, Let the carry digit in the hundred digit be $x$. Then $9 S+8=10 x+b$ $\qquad$
$9 b+x=S$ $\qquad$ (2); $x=S-9 b$ $\qquad$ (3)

Sub. (3) into (1): $9 s+8=10(S-9 b)+b \Rightarrow 8=S-89 b$
$$
\Rightarrow S=8, b=0
$$