Krystiana earns $15 x 3 = $<<15*3=45>>45 for the rooms on the first floor every month.
She earns $20 x 3 = $<<20*3=60>>60 for the rooms on the second floor every month.
And the cost for each room on the third floor is $15 x 2 = $<<15*2=30>>30.
So, the total amount she earns for the rooms on the third floor is $30 x 2 = $<<30*2=60>>60 every month.
Therefore, the total amount she earns from the apartment building is $60 + $60 + $45 = $<<60+60+45=165>>165.
#### 165