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GRE Counting and Probability: Practice Question of the Week #39

Here’s this week’s practice question– good luck, we’ll be posting the answer tomorrow!

How many integers between 1 and 10^21 are such that the sum of their digits is 2?

  1. 190
  2. 210
  3. 211
  4. 230
  5. 231

Update: Answer/explanation post!

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4 Responses to GRE Counting and Probability: Practice Question of the Week #39

  1. SKJ February 20, 2012 at 8:59 pm #

    231

    one digit nos adding to 2 (only 2) => 1
    2 digit nos(11,20) => 2
    3digit nos(101,110,200) => 3
    4 digit nos(1001,1010,1100,2000) => 4

    similarly
    ————————————–
    20 digit nos will be =>20
    21 digit nos will be =>21
    Answer is 1+2+3+…….+20+21 = 231

  2. Ravi February 20, 2012 at 1:50 pm #

    231 only if you consider two 1’s and 251 if you include two 1’s and one 2

  3. Ravi February 20, 2012 at 1:45 pm #

    1+2+3+….+21 = 231

  4. Ravi February 20, 2012 at 1:44 pm #

    231


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