Bunuel
If a positive integer is equal to the following product: \(2^5b^3c^4\), where b and c are distinct prime numbers greater than 2, how many distinct even factors does the integer have?
A. 1
B. 5
C. 50
D. 100
E. 120
Are You Up For the Challenge: 700 Level QuestionsSolution:
• \(b\) and \(c\) are distinct prime numbers greater than \(2\).
o We know that any prime number greater \(2\) is an odd number.
\(b\) and \(c\) are odd prime numbers.
Now, there are two ways to solve the problem, one is to find total factors and odd factors of the product and subtract odd factors from total factors, and the second way is to find even factors directly using the factor formula but many students commit the mistake in this method.
Method 1: \(2^5*b^3*c^4\)
• Total factors = \((power\ of\ 2 +1)*(power\ of\ b+1)*(power\ of\ c+1)=(5+1)*(3+1)*(4+1)=6*4*5=120\)
• Odd factors = \((Power\ of\ b+1)*(power\ of\ c+1)=(3+1)*(4+1)=4*5=20\)
o Even factors = \(120-20 = 100\).
Method 2:
Note:- while finding odd factors we completely ignore the power of even prime, but we can’t ignore the power of odd prime while calculating the even factors because \(even*odd = even\)
• Even factors = \((power\ of\ 2)*(power\ of\ b +1)*(power\ of\ c + 1)=(5)*(3+1)*(4+1)=5*4*5=100\).
Hence, the correct answer is
Option D.