Delete (Reverse Backspace) Button
Name your variables and fields meaningfully. I have no idea what a,b,e, etc are.  
Don't split your variable declaration and initalization. Instead of


final EditText ed;

Button dlt;

dlt = (Button) findViewById(R.id.button1);

ed = (EditText) findViewById(R.id.editText1);



do 

final EditText ed = (EditText) findViewById(R.id.editText1);    
final Button dlt = (Button) findViewById(R.id.button1);


And be consistent with your final usage. Either use it everywhere where appropriate or don't. Otherwise I start to wonder why ed is final and dlt is not. 
There a TextUtils.isEmpty() method available on Android. Use it instead of 


a11 != null &amp;&amp; !a11.trim().equals("")


Use static field access. Instead of t1.LENGTH_SHORT use Toast.LENGTH_SHORT. All the fields t,t1 and t2 are unnecessary.
All your fields should be local variables.