Find the measure of ∠C if
∠A = 3 2 x + 20
∠B = 7 2 x + 20
∠C = 5 2 x + 40
∠D = 3 2 x + 10
∠A = 3 2 x + 20
∠B = 7 2 x + 20
∠C = 5 2 x + 40
∠D = 3 2 x + 10