In fact, both Amanda's and Stephen's profs are correct; they are just using different supplementary angles. Amanda's is using the supplementary angles <span>∠1 and ∠4, and</span> ∠3 and ∠4, whereas Stephen is using ∠1 and ∠2, and <span>∠2 and ∠3.</span><span> </span>Please check the picture to visualize this more effectively.
