How To Take Data From Edit Text And Times It Against A Spinner Value - Dynamic Update

I'm trying to make a simple order system. So the user selects a burger type from a spinner, then the user can enter in the quantity and then there is some math taking place in the background which sets a text view at the bottom with the total price. I'm not sure what method I'm meant to use so that it is updated dynamically when the user changes the text view quantity.

If i just say that all burgers are 2.50, i need some sort of method saying, if/when data is entered into the edit text , take that data and then times it by 2.50 and set it in the total text view below. I'm just unsure on the method part of it so it's dynamic and will change when the edit text quantity is altered.

Code:
package placeorder.com;
import java.util.Random;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
[code]...