To find the E. coli population 4 hours after the culture was started, we first need to convert the time from hours to 20-minute increments.
There are 60 minutes in an hour, so:
- In 4 hours, there are \(4 \times 60 = 240\) minutes.
- Since each increment is 20 minutes, we divide the total minutes by 20: \[ \frac{240}{20} = 12. \] So, \(t = 12\) increments of 20 minutes.
Now, we can use the function \(f(t) = 10 \cdot 2^t\) to find the population: \[ f(12) = 10 \cdot 2^{12}. \]
Next, we calculate \(2^{12}\): \[ 2^{12} = 4096. \]
Now, we multiply by 10: \[ f(12) = 10 \cdot 4096 = 40960. \]
Thus, the E. coli population 4 hours after the culture was started is \(40,960\).
Therefore, the correct response is: 40,960.