To find the missing number \( x \) in the equation \( 80% \text{ of } x = 56 \), you can set up the equation as follows:
\[ 0.8x = 56 \]
Now, solve for \( x \) by dividing both sides of the equation by 0.8:
\[ x = \frac{56}{0.8} \]
Calculating the right side gives:
\[ x = 70 \]
So, the missing number is 70.