In figure, if AB || CD, CD || EF and y : z = 3 : 7, find x.

∵ AB || CD
and CD || EF
∴ AB || EF
| Lines parallel to the same line are parallel to each other
∴ x = z    ...(1)
| Alternate Interior Angles
x + y = 180°    ...(2)
| Consecutive interior angles on the same side of the transversal GH to parallel lines AB and CD
From (1) and (2),
z + y = 180°
y : z = 3 : 7
Sum of the ratios = 3 + 7 = 10