Logic trainer, fill the boxe >summation of two previous numbers

The program adds a couple (two nums) and stores them in the field above them. it continues till the last field is filled. For example, the question mark right next to 6 plus 6 equals 10 (i.e. 6+x = 10) .Use of arrays is not allowed. Use loops like for, while and getchar() ( or a better way(?) )to control the input (user enters 2 ? ? ?... or ? ? 10 ? 6... and so on). Any help on writing the code or the code itself (completely) is deeply appreciated. I need it for tomorrow afternoon. Thank you in advance for your time.

?
? 10
? 6 ?
2 ? ? ?

We won't do your assignment for you, post the code you've come up with so far.