Predict how many students in the school have at least one grandparent who belongs to a social network

80-90 students have at least one grandparent who belongs to a social network

Because they are parallel lines intersected by a common line, they make the same angle in the same parts. So angle HFC and DBC are the same. Therefore you can set them equal to each other and solve. 

5x = 3x +40 > Subtract 3x from both sides
2x = 40 > divide both sides by 2
x = 20

So, the angle HFC must be 100 degrees.