Let's call the initial number of bacteria in the petri dish as $N_0 = 100$. After 12 hours, the number of bacteria triples, which means there are now $3N_0$ bacteria. After another 12 hours, the number of bacteria triples again, which means there are now $3(3N_0) = 9N_0$ bacteria.
We can see a pattern here that after every 12 hours, the number of bacteria is multiplied by 3. Let's calculate the number of bacteria after 1 hour:
$\sf\implies\:N_1 = N_0 \times 3^{1/12}$
After simplifying:
$\sf\implies\:N_1 = 100 \times 3^{1/12}$
Using a calculator, we can find that $\sf\:3^{1/12} \approx 1.1548$. Therefore:
$\sf\implies\:N_1 \approx{\boxed{115.48}}$
So the equivalent hourly rate at which the number of bacteria is increasing is approximately 15.48% per hour.
In general, if the number of bacteria triples every $t$ hours, the equivalent hourly rate can be calculated as:
$\sf\implies\:r = 3^{1/t} - 1$
where $r$ is the hourly rate expressed as a decimal.
