Final answer:
The mathematical sentence to represent the total duration of Franklin's interviews for his documentary is 'f=6*5', where 'f' is the duration in minutes.
Step-by-step explanation:
To translate the student's problem into a mathematical sentence using 'f' as the variable, we consider the number of interviews conducted by Franklin and the duration of each interview. Each interview lasts for 5 minutes and he conducts 6 interviews in total. The total duration for all interviews in minutes can be represented by the mathematical sentence f=6*5. This means that to find the total time, f, for the documentary, we simply multiply the number of interviews by the duration of each interview.