I thought that nouns after propositions were not supposed to be given the definite article? de ce nu e ”mașină?”
Because of the modifier: "de spălat" ("for washing"), which indicates the type of machine we're speaking of.
If the sentence was "Ea pune prosopul în mașină" that would most likely mean "She puts the towel in the car."
Can't wait until I can use "cars of washing"
Washing machine can be just "washer".
The rising intonation makes this a question, to my ear.