to see if a pair of ratios is proportional... you need to compute for its cross products. If the cross products are equal then they are proportional.
a/b = c/d where ad = bc
a) 4/9 = 16/45 ⇒ 445 = 916 ⇒ 180 = 144 NOT PROPORTIONAL
b) 5/12 = 25/144 ⇒ 5144 = 1225 ⇒ 720 = 300 NOT PROPORTIONAL
c) 12/18 = 18/27 ⇒ 1227 = 1818 ⇒ 324 = 324 PROPORTIONAL
d)15/20 = 7/10 ⇒ 1510 = 207 ⇒ 150 = 140 NOT PROPORTIONAL
