When Equations look like One-way Streets

The following question (copied verbatim) was presented by another teacher to a class I was sitting in on, and students were asked "What is the answer?"

7 + 3 = _____ + 5 = _____ + 6 = _____

One student, whom we'll call *Xander, answered "21, because 7 + 3 = 10 + 5  = 15 + 6 = 21"

What makes sense about Xander’s answer?

What does Xander's answer tell us about his thinking?

What would you say/do next?

*Names have been changed.