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 ⇒ 4*45 = 9*16 ⇒ 180 = 144 NOT PROPORTIONAL
b) 5/12 = 25/144 ⇒ 5*144 = 12*25 ⇒ 720 = 300 NOT PROPORTIONAL
c) 12/18 = 18/27 ⇒ 12*27 = 18*18 ⇒ 324 = 324 PROPORTIONAL
d)15/20 = 7/10 ⇒ 15*10 = 20*7 ⇒ 150 = 140 NOT PROPORTIONAL
