Bunuel wrote:

P* is defined as the number of positive even integers less than P, if P is odd. If P is even, P* is defined as the number of prime integers less than P. What is (5* + 10*)*?

(A) 3*

(B) 4*

(C) 7*

(D) 10*

(E) 11*

Since 5 is odd, 5* represents the number of even integers less than 5. Since the even integers less than 5 are 4 and 2, 5* = 2.

Since 10 is even, 10* represents the number of primes less than 10. Since the primes less than 10 are 7, 5, 3, and 2, 10* = 4.

So, (5* + 10*)* = (2 + 4)* = 6* = 3, since 6 is even and the primes less than 6 are 5, 3, and 2.

We need to find an answer choice that is equal to 3.

We see that 7* = 3, since there are 3 even numbers less than 7, namely, 6, 4, and 2.

Answer: C

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