The decimal parts of two irrational numbers in ( 0 , 1 ) taken at random are found to 2 0 1 4 places. The probability that the smaller one can be subtracted from the other without borrowing can be expressed as p k , where p is a rational number and k > 1 0 0 0 is a positive integer. Find the last three digits of ⌊ 1 0 0 ( p + k + 1 ) ⌋ .
Assume for this question that the digit in first decimal place of the larger number is greater than the digit in that of the other. (As a follow up, can you say what would the probability be, without this assumption?)
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But the question asks for ⌊ 1 0 0 ( p + k + 1 ) ⌋ ...
EDIT: Okay I see below
(Even with your edit of k = 2 0 1 3 )
Why must be have p = . 5 5 and k = 2 0 1 3 ?
Why can't we have p = . 5 5 3 and k = 3 2 0 1 3 ?
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That is a problem I often face while mending a question for accepting numerical answers, thanks Sir. Let's say k > 1 0 0 0 .
Since α ≥ β , wouldn't we have a 1 ≥ b 1 by default?
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Yes, you are correct. We cannot have 1 0 0 choices for { a 1 , b 1 } . There are 5 5 valid and possible choices for this pair. So k = 2 0 1 3 basically. The question has been changed. Thanks for noticing,
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I hate to be a pain but, if a 1 = b 1 , Then we have a 2 ≥ b 2 . And as long as a j ≤ b j for all 1 ≥ j < i , We have a i ≥ b i for i ≤ 2 0 1 4
And your original question would have been correct ( the ( 1 0 0 5 5 ) 2 0 1 4 one ) if you replaced the clause "the smaller one" with "the first one".Sorry for the inconvenience (which I'm sure must be a lot).
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@Siddhartha Srivastava – When I thought of this question, it was to subtract one of them from the other. However, since subtracting a larger number from a smaller one, has complex relations with respect to taking borrow & so, so I changed the question a little. But I see the same problem is arising. I am adding a detail.
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Let the two randomly chosen irrational numbers be α = 0 . a 1 a 2 a 3 ⋯ a 2 0 1 4 and β = 0 . b 1 b 2 b 3 ⋯ b 2 0 1 4 where a i , b i ∈ { 0 , 1 , 2 , ⋯ , 9 } for 1 ≤ i ≤ 2 0 1 4 . Suppose α > β . For subtraction of β from α without borrowing, a i ≥ b i .
Therefore, for 1 ≤ i ≤ 2 0 1 4 , a i ∈ { b i , b i + 1 , ⋯ , 9 } . Thus after choosing b i , a i can be chosen in ( 1 0 − b i ) ways. So the number of favourable cases, given any b i , is ∑ j = 0 9 ( 1 0 − j ) = ∑ j = 1 1 0 j = 5 5 . The total number of ways of selection of a i and b i is 1 0 × 1 0 = 1 0 0 .
Thus the required probability is (for 2 0 1 4 digits) = ( 1 0 0 5 5 ) 2 0 1 4 . Thus 1 0 0 ( p + k ) = 1 0 0 ( 0 . 5 5 + 2 0 1 4 ) = 2 0 1 4 5 5 . ■