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Olympiad workbook
Exam
Olympiad workbook
Subject
IOQM
Chapter
Basics of Mathematics

Olympiad workbook · IOQM · Basics of Mathematics

For a natural number n , let n^ denote the number obtained by...

For a natural number nn, let n′n^{\prime} denote the number obtained by deleting zero digits, if any. (For example, if n=260,n′=26n=260, n^{\prime}=26; if n=2020n=2020, n′=22n^{\prime}=22.) Find the number of 3 -digit numbers n for which n′n^{\prime} is a divisor of nn, different from nn.

Correct answer

93

Step-by-step solution

\because n^{\prime} \neq nsoatleastonedigitofamustbezeroLetso atleast one digit of a must be zero Letn=\overline{a b c}CaseI.IfonlyCase I. If onlyc=0 \Rightarrow \overline{a b} \mid \overline{a b c} \Rightarrow \overline{a b} \mid 10 \overline{a b}(alwaystrue)Numberofnumbers(always true) Number of numbersn=9 \times 9=81CaseII.IfonlyCase II. If onlyb=0 \Rightarrow \overline{a c} \mid \overline{a b c} ⇒10a+c∣100a+c⇒10a+c∣10a−8c\begin{aligned} & \Rightarrow 10 a+c \mid 100 a+c \\ & \Rightarrow 10 a+c \mid 10 a-8 c \end{aligned}SoeitherSo either10 a=8 coror8 c-10 a=\lambda(10 a+c)forsomenaturalnumberfor some natural number\lambda.When. When10 a=8 c \Rightarrow 5 a=4 c \Rightarrow(a, c)=(4,5)WhenWhenc(8-\lambda)=10 a(\lambda+1) \Rightarrow c=a[908−λ−10]\left[\frac{90}{8-\lambda}-10\right] for λ=1,c=a\lambda=1, c=a\left(\frac{20}{7}\right)(Notpossible)for(Not possible) for\lambda=2, c=5 a \Rightarrow(a, c)=(1,5)forfor\lambda=3, c=8 a \Rightarrow(a, c)=(1,8)forfor\lambda=4, c=a(252)\left(\frac{25}{2}\right) (Not possible) and no further value of λ\lambda is needed to be checked Number of numbers n=3n=3, which are 405,105 and 108. Case III. If both b=c=0b=c=0, then all values of nn are acceptable. Number of numbers n=9n=9 So total number of possible values of n=93n=93.
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