So 6 in.w x 4 inches in.h
what will it be 30 inches wide

what we are doing right now is putting height over width, then the same thing to the other height over the width
now we cross multiply
(4) (30) = (6)(x)
120 = 6 x
divide both sides by 6 to isolate the variable
20 = x
The height of the poster, that has the width of 30 inches, will be 20 inchesHope this helps :)