chetan2u wrote:
What is the probability that the sum of two different single-digit prime numbers will NOT be prime?
(A) 0
(B) \(\frac{1}{2}\)
(C) \(\frac{2}{3}\)
(D) \(\frac{5}{6}\)
(E) 1
Diagnostic # 11
-------CONCEPT------------
Single digit prime numbers are
2,3,5,7
Now, except 2 all are odd so we can only get an odd sum if one of our prime number is even while other is odd (Only an odd number can be prime).
2+3 = 5
2+5 = 7
are only 2 cases to satisfy the condition.
while 2 + 7 = 9, it is not prime
& Now the total cases to get a sum by selecting any 2 prime numbers out of these 4 are 4C2 = 6
So, probability for sum NOT to be prime number = 4/6 = 2/3