SAT Math Multiple Choice Practice Question 160: Answer and Explanation

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Question: 160

6. 1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 . . .

The number above begins with a "1", then continues "12," "123," "1234," and so on until the number "9" is reached. How many digits are in the final number?

A. 36
B. 40
C. 45
D. 54
E. 81

Correct Answer: C

Explanation:

C. You probably noticed that the length of each string of numbers is the same as the last number: For example, the string ending in 4 is four numbers long. That means the final number will have 1 + 2 + 3 + . . . + 9, or 45, digits.

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