Firstly, I wrote down the various addition sums that make each of the number and my plan was to methodically work through the possible order of the cards by starting with either the 3 or the 14 envelope as these only had two options and writing them down and using trial and error.
By coincidence or maybe sub-conscious knowledge, I actually wrote down a possible solution for the number straight away.
3=2+1
7=7+0
8=5+3
13=4+9
14=6+8
However, I then looked again at the problem and asked myself ‘What if there is more than one way of making the numbers in envelopes work?’ By working through the options for 8, and using trial and error to place the numbers only once, I worked out that the 8 envelope could actually hold 5+3, 8+0 or 7+1 but it cannot hold 6+2.