A student has a collection of stamps that he wants to number starting from ‘1’. He used a total of 912 digits for this task. How many stamps are in his collection?

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Correct answer marked below

321

340

Correct

420

912

1028

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Official Explanation

Official

To find the number of stamps, we first calculate the digits used for numbering. From 1 to 9, there are 9 digits used. From 10 to 99, there are 90 numbers, each using 2 digits, totaling 180 digits. So far, we have used 9 + 180 = 189 digits.…

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What this question tests

What it tests

Whether you can count arrangements and selections correctly — permutations vs. combinations.

Common trap

Overcounting: treating a selection as ordered when order does not matter, or forgetting to divide by duplicates.