If m∠4 = (3x)° and m∠8 = (x + 40)°, what is the measure of ∠4? ∠4 and ∠8 are vertical.

Vertical angles are the angles opposite by the vertex.

The vertical angles have the same measure.

So, m ∠ 4 = m ∠ 8

=> 3x = x + 40

=> 3x - x = 40

=> 2x = 40

=> x = 40 / 2 = 20

Then, m ∠ 4 = 3x = 3(20°) = 60°.

60°