I have this situation:

```
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"  
android:layout_width="fill_parent"  
android:layout_height="fill_parent"  
>  

<ImageButton android:id="@+id/btn1"  
 android:layout_width="60px"   
    android:layout_height="80px"  
    android:layout_marginLeft="50px"
    android:layout_marginTop="50px"
    android:clickable="true"  
    android:tag="1"  
    android:background="@drawable/xo" 
/>  

<ImageButton android:id="@+id/btn2"  
 android:layout_width="60px"   
    android:layout_height="80px"  
    android:layout_marginLeft="120px"
    android:layout_marginTop="50px" 
    android:clickable="true"  
    android:tag="2"  
    android:background="@drawable/xo" 
/>  

<ImageButton android:id="@+id/btn3"  
 android:layout_width="60px"   
    android:layout_height="80px"  
    android:layout_marginLeft="190px"
    android:layout_marginTop="50px"   
    android:clickable="true"  
    android:tag="3"  
    android:background="@drawable/xo" 
/>  
</RelativeLayout>

```

Can I use percents instead of 50px? If yes how is the syntax?
I have this 3 buttons, and want to display in the middle of the page. 
I want to be one after the other, aligned horizontally.