Question 1023764
A) An even number
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That's one of these ten: 

2,4,6,8,10,12,14,16,18,20

out of these twenty: 

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19.20

That's 10 ways out of 20, so we make the fraction
{{{10/20}}} and reduce it to {{{1/2}}}
Answer: {{{1/2}}}

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B) A factor of 24 
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That's one of these seven: 

1,2,3,4,6,8,12

out of these twenty: 

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19.20

That's 7 ways out of 20, so we make the fraction
{{{7/20}}} and it doesn't reduce
Answer: {{{7/20}}}
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C) A prime number
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That's one of these eight: 

2,3,5,7,11,13,17,19

out of these twenty: 

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19.20

That's 8 ways out of 20, so we make the fraction
{{{8/20}}} and reduce it to {{{2/5}}}
Answer: {{{2/5}}}

Edwin</pre>