To find out how many gallons of perfume will be in each container, we need to divide the total amount of perfume by the number of containers.
First, we convert the number of containers: \[ 3.5 \times 10^2 = 350 \]
Now, divide the total gallons of perfume (10.5 gallons) by the number of containers (350): \[ \text{Gallons per container} = \frac{10.5}{350} \]
Calculating this gives: \[ \frac{10.5}{350} = 0.03 \]
Now we need to express 0.03 in scientific notation. We can write: \[ 0.03 = 3 \times 10^{-2} \]
Therefore, the answer is: \[ \boxed{3 \times 10^{-2}} \]