Let \( t \) be the number of seconds after the filling starts.
First Vat
The first vat starts with \( 121212 \) gallons and is being filled at a rate of \( 191919 \) gallons per second. Thus, the amount of liquid in the first vat after \( t \) seconds can be represented as:
\[ \text{Amount in first vat} = 121212 + 191919t \]
Second Vat
The second vat starts with \( 404040 \) gallons and is being filled at a rate of \( 151515 \) gallons per second. The amount of liquid in the second vat after \( t \) seconds can be represented as:
\[ \text{Amount in second vat} = 404040 + 151515t \]
Setting the Amounts Equal
We need to find \( t \) such that both vats contain the same amount of liquid:
\[ 121212 + 191919t = 404040 + 151515t \]
Solving for \( t \)
Rearranging the equation:
\[ 191919t - 151515t = 404040 - 121212 \]
This simplifies to:
\[ 40304t = 282828 \]
Now, we can solve for \( t \):
\[ t = \frac{282828}{40304} \]
Calculating:
\[ t = 7 \]
Final Result
Thus, both vats will have the same amount of liquid after \( \boxed{7} \) seconds.