<span>Answer:
Since X is the midpoint, you know WX=XY and VX=XZ (given to you in the problem statement)
When you draw your line segments, label the five points W,X,Y,V and Z
Now also draw line segments WV and ZY.
You now have two triangles.
The angle zxy = wxv since opposite angles are equal.
Given the above, you now have 2 triangles that are congruent using side-angle-side
Since the two triangles are congruent, then the sides opposite the angle x (WV and ZY) must be equal.</span>

If 2x+y=5, then y=5-2x, so he substituted 5-2x correctly.
x+4x=5x, so he combined like terms correctly.
If 5x-10=10, then 5x=10+10 -> 5x=20, so he subtracted 10 from the right side instead of adding 10 to the right side.
The answer is C.
Here's the correct solution:
Let the price of the 3rd card be c. Now we can write the equation:
c + (c + 20) + (c + c + 20) = 160 .........(1)
Simplifying the left hand side of equation (1), we get:
4c + 40 = 160 .........(2)
Subtracting 40 from both sides of equation (2) gives:
4c = 120
from which c = $30.
Therefore the price of the second card = 30 + 20 = $50
and the price of the first card = 30 + 50 = $80.