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I started off by creating a table of all of the possible number bonds to each of the targets on the envelopes.

I then worked systematically so that I tried every potential pathway across the table. Some of them proved to be impossible but I was able to find all 3 solutions. I have also included an example of a pathway that was impossible; it was where I started the process.

I know that there are only 3 possible solutions as I worked through every possibility in a systematic fashion.