Answer:
15
Step-by-step explanation:
Single-digit integers have one digit each. Among the numbers 1-12, there are 9 of these: 1-9.
Two-digit integers have two digits each. Among the numbers 1-12, there are 3 of these: 10-12.
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The total number of digits Rena needed for writing the 12 integers was ...
9×1 +3×2 = 15 . . . digits
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Of course, there were 10 unique digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.
