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IMO - E


numbers divisible by 3 : 12,15,18........99 i.e. 30 terms
numbers divisible by 7 : 14.......98 i.e. 13 terms

common terms multiples of 3&7 which counted twice. LCM of 3,7 is 21 so no terms are 21,42,63,84 i.e. 4 terms

Total numbers divisible by 3 or 7 = 30+13-4= 39 ; total 2 digit numbers = 90

Hence answer is 39/90.
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Hello,

Can someone explain to me why the answer should not be 35/90?

This is a probability question and the question states to find probability of selecting a number that is divisible by 3 OR 7 and not 3 AND 7.

As such, total multiples of 3 = 30
total multiples of 7 = 13

Subtract 4 common multiples (twice - since counted in both 3 and 7) = 30+13-8 = 35 --> probability = 35/90.

Kindly explain why the above is wrong? Thanks!
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Hello,

Can someone explain to me why the answer should not be 35/90?

This is a probability question and the question states to find probability of selecting a number that is divisible by 3 OR 7 and not 3 AND 7.

As such, total multiples of 3 = 30
total multiples of 7 = 13

Subtract 4 common multiples (twice - since counted in both 3 and 7) = 30+13-8 = 35 --> probability = 35/90.

Kindly explain why the above is wrong? Thanks!

The “or” here is an inclusive “or,” which means numbers divisible by 3, by 7, or by both.
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