If \(a, b, c , d, e,\) and \(f\) are positive integers and \(\frac{a}{b}−\frac{c}{d}−\frac{e}{f}=0\), is \(f\) an even integer ?
(1) The highest common factor of \(b\) and \(d\) is odd
The highest common factor of \(b\) and \(d\) is odd implies that either \(b\) and \(d\) is odd or one of \(b\) and \(d\) is even
For example highest common factor of \(3\) and \(5\) is \(1 (odd)\), in this case, \(\frac{a}{3}−\frac{c}{5}−\frac{e}{f}=0\)
\(\frac{5a-3c}{15}=\frac{e}{f}\) Here, \(f\) is odd
For example highest common factor of \(6\) and \(3\) is \(3 (odd)\), in this case, \(\frac{a}{6}−\frac{c}{3}−\frac{e}{f}=0\)
\(\frac{a-2c}{6}=\frac{e}{f}\) Here, \(f\) is even
-Not Sufficient
(2) The least common multiple of \(e\) and \(d\) is odd
The least common multiple of \(e\) and \(d\) is odd implies that both \(e\) and \(d\) is odd
For example least common multiple of \(3\) and \(5\) is \(15 (odd)\), in this case, \(\frac{a}{b}−\frac{c}{5}−\frac{3}{f}=0\)
\(\frac{5a-bc}{5b}=\frac{3}{f}\) Here, \(f\) is odd
Here, if \(a=1\), \(b=2\) and \(c=1\) then \(f=10\) (even)
But if \(a=1\), \(b=1\) and \(c=2\) then \(f=5\) (odd)
-Not Sufficient
Combining (1) and (2) \(b=even/odd, d=odd, e=odd\)
when \(b=even, d=odd\) and \(e=odd\)
\(\frac{a}{even}−\frac{c}{odd}=\frac{odd}{f}\), \(f=even\)
when \(b=odd, d=odd\) and \(e=odd\)
\(\frac{a}{odd}−\frac{c}{odd}=\frac{odd}{f}\), \(f=odd\)
Answer : E