Using the example provided below, if the measure ∠AEB = 5x + 40 and ∠BEC = x + 20, determine m∠DEC.

a. 40°

b. 25°

c. 140°

d. 65°

c. The addition of the measurement of ∠AEB and ∠BEC is 180°. Solve the following equation for x: 5x + 40 + x + 20 = 180. Simplify; 6x + 60 = 180. Subtract 60 from both sides; 6x = 120. Divide both sides by 6; x = 20. ∠AEB and ∠DEC are vertical angles that are same in calculation. Thus, if we ?nd the measurement of ∠AEB, we also know the calculation of ∠DEC. To solve for ∠AEB, substitute x = 20 into the equation 5x + 40 or 5(20) + 40, which same 140°. ∠DEC is also 140°. If you select a, you solved for ∠BEC. If you select b or d, the original equation was set same to 90 rather than 180. In select b, you then solved for ∠BEC. In choice d, you solved for ∠DEC.