only static member be accessed in initializers in flutter - flutter

Question

I want to try to define below code but when I write it, show me the Error only static member be accessed in initializers, the problem is I can't define it anywhere, the code that I want to define is here, I need to use to calculate discount
double discount=(prod_old_priceprod_old_price-prod_price)/prod_old_price*100;
and this my project code:
class ProductDetails extends StatefulWidget {
final prod_fullName;
final prod_pic;
final prod_old_price;
final prod_price;
ProductDetails({
this.prod_fullName,
this.prod_pic,
this.prod_old_price,
this.prod_price,
});
children: <Widget>[
Expanded(
child: MaterialButton(
onPressed: () {},
color: Colors.white,
textColor: Color(0xff989898),
elevation: 0.2,
child: Row(
children: <Widget>[
Expanded(child: new Text('Color')),
Expanded(child: new Icon(Icons.arrow_drop_down)),
],
),
),
),
Expanded(
child: MaterialButton(
onPressed: () {},
color: Colors.white,
textColor: Color(0xff989898),
elevation: 0.2,
child: Row(
children: <Widget>[
Expanded(child: new Text('Size')),
Expanded(child: new Icon(Icons.arrow_drop_down)),
],
),
),
),
Expanded(
child: MaterialButton(
onPressed: () {},
color: Colors.white,
textColor: Color(0xff989898),
elevation: 0.2,
child: Row(
children: <Widget>[
Expanded(child: new Text('Qty')),
Expanded(child: new Icon(Icons.arrow_drop_down)),
],
),
),
),
],
)
],
),
bottomNavigationBar: Material(
elevation: 7.0,
color: Colors.white,
child: Container(
height: 60.0,
width: MediaQuery.of(context).size.width,
color: Colors.white,
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceBetween,
children: <Widget>[
Padding(
padding: const EdgeInsets.only(right: 10.0),
child: InkWell(
onTap: (){},
child: Container(
height: 40.0,
width: MediaQuery.of(context).size.width - 280.0,
decoration: BoxDecoration(
color: Color(0xffff5f60),
borderRadius: BorderRadius.circular(10),
),
child: Center(
child: Text(
'add to cart',
style: TextStyle(color: Colors.white,fontSize: 20.0,fontWeight: FontWeight.bold),
),
),
),
),
),
Container(
decoration: BoxDecoration(
color: Color(0xfff40725),
borderRadius: BorderRadius.circular(50),
),
child: Padding(
padding: const EdgeInsets.all(5.0),
child: Text("----------------I WANT USE DISCOUNT HERE --------------------------",style: TextStyle(color: Colors.white,fontWeight: FontWeight.bold),),
),
),
Padding(
padding: const EdgeInsets.only(top:5.0,left:8.0),
child: Container(
child: Column(
children: <Widget>[
Text("\$${widget.prod_old_price}",textAlign: TextAlign.left,style: TextStyle(fontSize: 18.0,color: Color(0xff989898),decoration: TextDecoration.lineThrough),),
Text("\$${widget.prod_price}",style: TextStyle(fontSize: 18.0,fontWeight: FontWeight.bold)),
],
),
),
),
],
),
),
),
);