To find the ratio of the number of dominoes still standing to the total number of dominoes, we first need to determine how many dominoes are still standing.
Jessica set up a total of 39 dominoes, and 9 of them have fallen over. We can calculate the number of dominoes still standing by subtracting the fallen dominoes from the total:
\[ 39 - 9 = 30 \]
Now, we have:
- The number of standing dominoes = 30
- The total number of dominoes = 39
The ratio of the number of dominoes still standing to the total number of dominoes can be expressed as:
\[ 30:39 \]
To simplify this ratio, we find the greatest common divisor (GCD) of 30 and 39. The GCD of 30 and 39 is 3.
Now we divide both numbers in the ratio by their GCD:
\[ \frac{30}{3} = 10 \] \[ \frac{39}{3} = 13 \]
Thus, the simplified ratio of the number of dominoes still standing to the total number of dominoes is:
\[ 10:13 \]
Therefore, the final answer is:
\[ \boxed{10:13} \]
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