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What is the probability of a number from 1-25 being an odd number or a factor of 20? Here's my working out:

Odd numbers: 12/25 (1, 3, 5, 7, 9, 11, 13, 15, 19, 21, 23, 25)

Factors of 20: 6/25 (1, 2, 4, 5, 10, 20)

Both: 2/25 (1, 5)

P(odd number or factor of 15) = P(odd number) + P(fator of 20) - P(both)

= 12/25 + 6/25 - 2/25 = 16/25

16/25 was too long for MyMaths' input box so I converted it to a decimal:

16 / 25 = 0.64

That was incorrect.

Will
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    You just forgot 17. There are 13 odd numbers. But otherwise the reasoning is perfect. – guaraqe Jun 09 '14 at 15:11
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    Listing and counting is a nuisance. Half of the numbers to $24$ are even, and half are odd, so far $12$ odd. And then there is $25$, for a total of $13$. – André Nicolas Jun 09 '14 at 15:14

1 Answers1

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Thanks to the person who pointed out that I left out the 17. The answer is 0.68. It's always the simple things that I get wrong!

Odd numbers: 13/25 (1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25)

Factors of 20: 6/25 (1, 2, 4, 5, 10, 20)

Both: 2/25 (1, 5)

P(odd number or factor of 15) = P(odd number) + P(factor of 20) - P(both)

= 13/25 + 6/25 - 2/25 = 17/25 17 / 25 = 0.68

Will
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