I already know this answer form a practice assignement, but can you help me understand how it it calculated?

A client owns an apartment building with a fair market value of $250,000, an adjusted basis of $175,000, and a mortgage of $150,000. The client exchanges the building and $40,000 cash for a different apartment that has a fair market value of $220,000. The client assumes the $80,000 mortgage on the building to be acquired.

Which tax amount will the client realize as a result of the exchange?

$0 gain

loss$30,000

gain$45,000

gain$75,000