Kind of hard!

An integer from 100 through 999, inclusive is to be chosen at random. What is the probability that the number chosen will have 0 at least as 1 digit?


Please note that A,B,C,D and E are just choices and not variables or anything else

A. 1 900 \text{A.}\frac{1}{900} C. 90 900 \text{C.}\frac{90}{900} E. 271 1000 \text{E.}\frac{271}{1000} B. 81 900 \text{B.}\frac{81}{900} D. 171 900 \text{D.} \frac{171}{900}

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3 solutions

Because we are working with numbers in the triple digits, our numbers with at least one 0 will have that 0 in either the units digit or the tens digit (or both, though they will only be counted once).

We know that our numbers are inclusive, so our first number will be 100, and will include every number from 100 though 109. That gives us 10 numbers so far.

From here, we can see that the first 10 numbers of 200, 300, 400, 500, 600, 700, 800, and 900 will be included as well, giving us a total of:

10*9

90 so far.

Now we also must include every number that ends in 0. For the first 100 (NOT including 100, which we have already counted!), we would have:

110, 120, 130, 140, 150, 160, 170, 180, 190

This gives us 9 more numbers, which we can also expand to include 9 more in the 200’s, 300’s, 400’s, 500’s, 600’s, 700’s, 800’s, and 900’s. This gives us a total of:

9*9

81

Now, let us add our totals (all the numbers with a units digit of 0 and all the numbers with a tens digit of 0) together:

90+81

171

There are a total of 900 numbers between 100 and 999, inclusive, so our final probability will be:

171 900 \frac{171}{900}

Our final answer is D, 171 900 \large \color{#EC7300} \frac{171}{900}

Munem Shahriar
Oct 5, 2017

There are 999 99 = 900 999-99=900 numbers in the range from 100 100 to 999 999 inclusive. The number of three-digit numbers using only digits 1 1 through 9 9 inclusive is 9 × 9 × 9 = 729. 9 \times 9 \times 9=729. Thus the probability that a random three-digit number has no zeroes in its decimal representation is

9 × 9 × 9 900 = 9 × 9 100 = 81 % \frac{9\times 9 \times9}{900} = \frac{9 \times 9}{100} = 81\%

and so the probability that it has at least one zero is

100 % 81 % = 19 % 100\%-81\%=19\%

which is the same as

171 900 = 19 100 = 19 % . \boxed{\dfrac{171}{900}} = \frac{19}{100} = 19\%.

Let X X be a random variable denoting the number of zeroes in a 3 3 digit number.

We are asked to find P ( X 1 ) P(X\geq 1) ,

P ( X 1 ) = 1 P ( X = 0 ) P ( X = 0 ) = 9 3 900 There are 9 ways to choose a digit such that it is 0 P ( X 1 ) = 1 9 3 900 = 171 900 \begin{aligned} P(X\geq1) &=1-P(X=0)\\ P(X=0)&=\dfrac{9^3}{900}\hspace{4mm}\color{#3D99F6}\text{There are } 9\text{ ways to choose a digit such that it is}\neq0\\ \implies P(X\geq1)&=1-\dfrac{9^3}{900}\\ &=\color{#EC7300}\boxed{\color{#333333}\dfrac{171}{900}}\end{aligned}

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