a. [tex] \frac{5}{30} = \frac{1}{6}[/tex] Equal
b. [tex] \frac{4}{12} = \frac{21}{60}[/tex] Not equal
c. [tex] \frac{17}{34} = \frac{41}{82}[/tex] Equal
d. [tex] \frac{6}{9} = \frac{25}{36}[/tex] Not equal
To cross-multiply you must, you must multiply the numerator (x/y) of each fraction by the denominator (x/y) of the opposing fraction.