Answer:
9
Step-by-step explanation:
lets do calculations with ONLY the rightmost digits, ie., 7 + 6 = 4, so we're ignoring the carry.
Then, following must be true as well:
7+5 = 3
7+4 = 2
7+3 = 1
7+2 = 0 <= this reveals our base 9
7+1 = 8
7+0 = 7
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