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It is not intuitive that the app would display fewer machines than are in the laundry room; this application does not display any information about dryers that are currently in use. But, displaying useful machines as a fraction of available machines would add extra text and complexity. In addition, the full/empty state of a machine might not be clear from the icon given. It could be that the visually similar icons will be enough for a user to tell a full dryer from an empty one, but a more universal way to communicate this idea might also be possible.
Some good points about the learnability of this design are its external consistencies with other Android applications and with itself. The app uses the default icons for text Messaging and Phone calls, providing good external consistency within the realm of mobile applications. The user would probably recognize these icons and intuit their functions accordingly. Also, the application displays internal consistency by having the same screen perform multiple functions. The home screen also acts as the display of time remaining, once the user's laundry is in a machine. Also, Another point of consistency is that the interface for reading notes is very similar to the interface for writing them. In order to get the user to recognize the text box for entering their new note, some highlighted default text could be used, along with bringing up the on-screen keyboard immediately upon entering this screen.
Efficiency
This application should be efficient to use. An expert user would immediately know how many machines are available, and be able to claim a machine quickly and without errors.
Safety