Answer:
"4.71" is the correct answer.
Explanation:
As we know, 45 envelopes addressed in 2 hours then,
⇒

and, 45 in 3 hours,
⇒

Now by adding all together, we get
⇒

⇒

and multiply it by "4.71", we get
⇒

⇒

Round it "310".
So that "4.71" is the right answer.