I think you forget to put Bundle
in Intent
like
intent.putExtras(b);
Correct with below:
Intent intent = new Intent(Examenes.this, Examen.class);
Bundle b=new Bundle();
b.putString("Inicio", txtTemaIn.getText().toString());
b.putString("Fin", txtTemaFin.getText().toString());
b.putString("Numero", txtNumPregs.getText().toString());
intent.putExtras(b);
startActivity(intent);