Hello!
I am new to App Inventor and I got stuck with a problem when I try to send a simple TEXT FROM APP to ARDUINO via BLUETOOTH.
It does not send the text 'a' nor 'b', but rather it sends other symbol of which I don't know as shown below:
HERE IS THE CODE for ARDUINO (a very simple code as it is)
and here is for the APP INVENTOR
and the RESULT from ARUIDNO Serial Monitor:
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I googled it for help, but couldn't find it so I write this to seriously seek for support.
Thank you so much for reading and helping 
Is the Serial Monitor in 9600?
Here examples:
ABG
3
Have you tried sending your 'a' and 'b' text using an off the shelf BlueTooth terminal program from the Play Store ?