36347

Answers:

Envelope: 7 8 13 14 3

calculations: 5+2 7+1 9+4 8+6 0+3

method:
First I got my pen and paper and wrote down all of the envelope numbers. underneath that I wrote down all of the possible combinations. After that, in the margin I wrote down the envelope numbers with a space underneath each one. Next to the first envelope number, I wrote down one of the possible calculations next to it like the following:
(envelope)(calculation)

7 5+2
(After that I carried on doing the same with all of the other solutions like the following) :
(this was my final result)

(Envelope) (Calculation)

7 5+2

8 7+1

13 9+4

14 8+6

3 0+3