To find the missing number \( x \) in the equation \( 0.1 \times x = 9 \), you can isolate \( x \) by dividing both sides of the equation by \( 0.1 \):
\[ x = \frac{9}{0.1} \]
Calculating that gives:
\[ x = 9 \div 0.1 = 9 \times 10 = 90 \]
So, the missing number is 90.