ISEE Upper Level Quantitative : How to find out if a number is prime

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #41 : How To Find Out If A Number Is Prime

\displaystyle x+ y = 12, and \displaystyle x and \displaystyle y are positive integers.

\displaystyle x is a prime number; \displaystyle y is not a prime number.

Which is the greater quantity?

(a) \displaystyle x

(b) \displaystyle y

Possible Answers:

(a) is the greater quantity 

It cannot be determined which of (a) and (b) is greater

(b) is the greater quantity 

(a) and (b) are equal

Correct answer:

It cannot be determined which of (a) and (b) is greater

Explanation:

\displaystyle x+ y = 12, and \displaystyle x and \displaystyle y are positive integers, so each of \displaystyle x and \displaystyle y is an integer from 1 to 11 inclusive.

\displaystyle x is a prime number, meaning that it can be equal to 2, 3, 5, 7, or 11. Testing each case:

\displaystyle x=2

\displaystyle 2+ y = 12

\displaystyle y = 10, which is not prime.

 

\displaystyle x= 3

\displaystyle 3+ y = 12

\displaystyle y = 9, which is not prime.

 

\displaystyle x= 5

\displaystyle 5+ y = 12

\displaystyle y = 7, which is prime - we throw this case out.

 

\displaystyle x= 7

\displaystyle 7+ y = 12

\displaystyle y = 5, which is prime - we throw this case out.

 

\displaystyle x= 11

\displaystyle 11+ y = 12

\displaystyle y = 1, which is not prime.

In the first two cases, \displaystyle y > x; in the last case, \displaystyle y < x. It cannot be determined which is the greater. 

Example Question #42 : How To Find Out If A Number Is Prime

\displaystyle x+ y = 10\displaystyle x and \displaystyle y are positive integers.

\displaystyle x is a prime number. \displaystyle y is not.

Which is the greater quantity?

(a) 5

(b) \displaystyle y

Possible Answers:

(a) and (b) are equal

It cannot be determined which of (a) and (b) is greater

(a) is the greater quantity 

(b) is the greater quantity 

Correct answer:

(b) is the greater quantity 

Explanation:

\displaystyle x+ y = 10, and \displaystyle x and \displaystyle y are positive integers.

Therefore, \displaystyle x must be an integer from 1 to 9, as must \displaystyle y.

Since \displaystyle x is prime, it can be any of 2, 3, 5, or 7.

Therefore, one of the following must hold:

\displaystyle 2+ y = 10

\displaystyle y = 8

 

\displaystyle 3+ y = 10

\displaystyle y = 7

 

\displaystyle 5+ y = 10

\displaystyle y = 5

 

\displaystyle 7+ y = 10

\displaystyle y = 3

 

Only in the first case is \displaystyle y not a prime number (8 has four factors - 1, 2, 4, 8), so \displaystyle x = 2 and \displaystyle y = 8 > 5.

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