ISEE top Level Quantitative help » Numbers and also Operations » factors / Multiples » prime Numbers » just how to discover out if a number is prime

Explanation:

Every number other than 1 contends least two determinants - 1 and also itself. So us are trying to find a number through no other components - the is, a prime.

You are watching: Prime numbers between 21 and 40

3 is a factor of both 33 and also 39, and 5 is a variable of 35, so we can remove those choices. Us cannot remove 37, due to the fact that it is a prime, having actually only 1 and also 37 together factors.

Which is the higher quantity?

(a) The variety of prime numbers in between 70 and 110

(b) The number of prime number between 80 and also 120

Explanation:

The primes in between 80 and 110 are had in both sets, so all we must do is to to compare the number of primes between 70 and also 80 and also the variety of primes between 110 and 120.

(a) The primes in between 70 and 80 room 71, 73, and 79 - three primes

(b) The only prime between 110 and also 120 is 113.

(a) is the greater quantity

Which is the higher quantity?

(a) The variety of prime numbers in between 1 and 20 inclusive

(b) The variety of composite numbers in between 21 and 30 inclusive

Explanation:

(a) The element numbers in between 1 and also 20 inclusive are 2, 3, 5, 7, 11, 13, 17, 19 - eight total.

(b) The prime numbers in between 21 and 30 inclusive space 23 and 29 - 2 prime numbers the end of ten integers. This leaves eight composite numbers.

(a) and also (b) are as such equal.

Which is the better quantity?

(a) The variety of prime numbers between 1 and 20 inclusive

(b) The variety of composite numbers between 1 and 20 inclusive

Explanation:

The element numbers in between 1 and 20 inclusive room 2, 3, 5, 7, 11, 13, 17, 19 - eight total. Because 1 is neither prime no one composite, this pipeline 11 composite numbers. (b) is the greater quantity.

Which is the greater quantity?

(a) The sum of the prime numbers between 11 and 19 inclusive

(b) The sum of the composite numbers between 11 and also 19 inclusive

Explanation:

(a) The prime numbers in the 11-19 range are 11, 13, 17, and 19. Your sum:

(b) The composite numbers in the 11-19 selection are 12, 14, 15, 16, and also 18. Their sum:

(b) is greater.

Which amount is greater?

(a) The number of composite numbers between 1 and also 1,000 inclusive.

(b) The number of even integers between 1 and 1,000 inclusive.

Explanation:

It is not necessary to count every one of the composite number in the 1-1,000 range. Other than for 2, every also number is composite, and there are more than one weird composite number (15, 25) to consist of for 2. This renders (a) greater.

Which is the better quantity?

(a) The variety of primes between 20 and also 50

(b) The number of primes in between 10 and also 40

Explanation:

The primes in between 20 and 40 are consisted of in both sets, so every we need to do is to to compare the variety of primes in between 40 and 50 with the number of primes in between 10 and 20.

(a) The primes in between 40 and 50 space 41, 43, and 47 - three.

(b) The primes in between 10 and also 20 are 11, 13, 17, and also 19 - four.

Since the variety of primes in between 10 and also 20 outnumbers those between 40 and also 50, the variety of primes in between 10 and also 40 outnumber those between 20 and also 50. Therefore, (b) is the greater.

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