Solution

Sealed Solution Archie

First name
Archie
School
Mountains Christian College
Country
Age
10

I started with 1 + 2 = 3 (envelope 1)
3+ 4 = 7 (envelope 2)
I left 8 ( envelope 3)
8 + 6 = 14 (envelope 5)
I realised I had numbers 9, 0, 5, 7 which I couldn't use for envelopes 4 or 3. Next attempt I used
3+ 0 - 3 (envelope 1) 1 + 7 = 8 (envelope 3)
5 + 2 = 7 (envelope 2) 8 + 6 again for envelope (5)
9 + 4 = 13 ( envelope 4)
Solution = 1+ 7 to make 8 for envelope 3