How do I have a specific action occur in the parent when navigating up from a child?

Question

I am very new to Android app programming and am trying to get a handle on some concepts through a simple calculator program that will contain a lot of useless features. Basically i have two EditText's (to enter two numbers) and a RadioGroup (to select the math operation to be performed on the numbers) in the main activity and I display the total in a child activity. This all works great. My problem is that when I hit the up button, I want the total to be displayed as the first EditText's hint in the MainActivity. Basically allowing the user to carry on a string of simple calculations without having to reenter the previous total. (the hint values are used if the EditText is left empty) I have tried onNavigateUpFromChild(Activity child), but to no avail. Is this even possible?

Here is my code thus far.

MainActivity.java:

public class MainActivity extends Activity {

                         //! Irrelevant code removed !//

    /** Variable Declaration */
    Float total; // Holds a running total
    EditText num1; // Text box for first number

                         //! Irrelevant code removed !//

    @Override
    public boolean onNavigateUpFromChild(Activity child) {
            num1.setHint(total.toString());
            return onNavigateUp();
    }

                         //! Irrelevant code removed !//
}

DisplayTotalActivity.java:

public class DisplayTotalActivity extends Activity {

                         //! Irrelevant code removed !//

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case android.R.id.home:
                NavUtils.navigateUpFromSameTask(this);
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.mdiehl.calculator"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="16"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

        <activity
            android:name="com.mdiehl.calculator.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.mdiehl.calculator.DisplayTotalActivity"
            android:label="@string/title_activity_display_total"
            android:parentActivityName="com.mdiehl.calculator.MainActivity"
            android:theme="@android:style/Theme.Holo" >
            <meta-data
                android:name="android.support.PARENT_ACTIVITY"
                android:value="com.mdiehl.calculator.MainActivity" />
        </activity>
    </application>
</manifest>

As far as I can tell, the onNavigateUpFromChild method is never even called.

Thanks in advance to anyone that can offer some assistance in this matter.