Firstly you start with number three as only the numbers one and two or three and zero fit inside it. If it is two and one then number seven could be either seven and zero or three and four. If three and zero is inside the first envelope then seven could be six and one or five and two. As soon as we come to the third envelope with the number eight on it we can eliminate the option of 3 and 0 as if we use that there is no way to make eight except for double four however we only have one four. Now we have got the numbers inside our first three envelopes.
Sum of 3: 1 and 2
Sum of 7: 0 and 7
Sum of 8: 3 and 5
Now we have the numbers four, six, eight and nine left with the sums of thirteen and fourteen. The only numbers that give us a sum of thirteen are four and nine whilst the only numbers that give us a sum of fourteen are six and eight. This means that the following table shows what numbers go into what cards.
3 = 1 and 2
7 = 0 and 7
8 = 3 and 5
13 = 4 and 9
14 = 6 and 8
Overall answer: the numbers three and five go into the envelope of the sum of eight.