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JAMB 1993 · UME · Q48

A number is selected at random between 20 and 30, both numbers inclusive. Find the probability that the number is a prime.

Worked solution (try it first)
  1. From 20 to 30 inclusive there are 11 numbers.
  2. The primes among them are 23 and 29.
  3. 21, 25 and 27 are odd but divisible by 3, 5 and 3.
  4. So the probability is 211\frac{2}{11}, option A.

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