This number isn't supposed to have the digit 7

Probability Level 2

If the probability that a number selected at random between 100 and 999 (both inclusive) does not contain the digit 7 can be expressed as a/b, where a and b are positive integers and co-prime.

Then what is a + b ?


The answer is 43.

Vijay Simha by Vijay Simha , 22, USA

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1 solution

X X
Feb 9, 2019

There are totally 900 numbers, and there are 8 × 9 × 9 8\times9\times9 numbers that doesn't contain 7.

Hence 8 × 9 × 9 900 = 18 25 \frac{8\times9\times9}{900}=\frac{18}{25}

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