Let \( t \) be the time in seconds after both vats have started being filled.
For the first vat, which starts with 14 gallons and is being filled at a rate of 20 gallons per second, the amount of liquid in the first vat after \( t \) seconds can be expressed as:
\[ \text{Amount in first vat} = 14 + 20t \]
For the second vat, which starts with 35 gallons and is being filled at a rate of 16 gallons per second, the amount of liquid in the second vat after \( t \) seconds can be expressed as:
\[ \text{Amount in second vat} = 35 + 16t \]
We want to find \( t \) such that both vats have the same amount of liquid:
\[ 14 + 20t = 35 + 16t \]
To solve for \( t \), we first will simplify the equation by subtracting \( 16t \) from both sides:
\[ 14 + 20t - 16t = 35 \]
This simplifies to:
\[ 14 + 4t = 35 \]
Next, subtract 14 from both sides:
\[ 4t = 21 \]
Now, divide both sides by 4:
\[ t = \frac{21}{4} = 5.25 \]
Thus, after \( t = 5.25 \) seconds, both vats will have the same amount of liquid.
To verify, we can calculate the amounts in both vats after 5.25 seconds:
- First vat:
\[ 14 + 20(5.25) = 14 + 105 = 119 \text{ gallons} \]
- Second vat:
\[ 35 + 16(5.25) = 35 + 84 = 119 \text{ gallons} \]
Since both vats have 119 gallons, the solution is confirmed.
The final answer is:
\[ \boxed{5.25} \] seconds.