C# winApp สุ่มค่าเพื่อให้ได้แบบที่ต้องการ ต้องทำยังไง ครับ

NumP = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100 };

#region _Draw Detail

            // System.Windows.Forms.MessageBox.Show("page " + iPage);
            int yC = 120, xC = 60;
            int w = 100, h = 40;
            for (int i = 0; i < 6; i++)
            {

                int a = RandomNumberGenerator.GetInt32(1, 50);
               
         
                int b = a * 100/NumP[RandomNumberGenerator.GetInt32(0, NumP.Count - 1)];
               /* int b = a * 100 / RandomNumberGenerator.GetInt32(1, 100);//a * Nums[RandomNumberGenerator.GetInt32(0, Nums.Count - 1)];
                do {
                    b++;
                } while (((Convert.ToDouble(a) * Convert.ToDouble(100) / Convert.ToDouble(b))%1)!=0);*/
                if (rd_1.Checked)
                {
                    string _a = a.ToString(), _b = b.ToString();


                    e.Graphics.DrawFraction(_a, _b, xC + 10, yC);
                    e.Graphics.DrawString(" = ", new Font("Angsana New", 20), new SolidBrush(Color.Black), xC + 110, yC + 15);
                    e.Graphics.DrawFraction("Y", "100", xC + 110, yC);
                }
                else
                {
                    if (RandomNumberGenerator.GetInt32(1, 1000) > 500)
                    {
                        e.Graphics.DrawString($"{a} คิดเป็นร้อยละเท่าใด\nของ {b}", new Font("Angsana New", 20), new SolidBrush(Color.Black), xC + 10, yC);
                    }
                    else
                    {
                        e.Graphics.DrawString($"ร้อยละ {a} ของ{b} \nเป็นเท่าใด", new Font("Angsana New", 20), new SolidBrush(Color.Black), xC + 10, yC);
                    }
                }
                
                e.Graphics.DrawLine(new Pen(Brushes.Black, 2), xC + 250, yC + 25, 700, yC + 25);
                e.Graphics.DrawLine(new Pen(Brushes.Black, 2), xC + 250, yC + 55, 700, yC + 55);
                e.Graphics.DrawLine(new Pen(Brushes.Black, 2), xC + 250, yC + 85, 700, yC + 85);
                e.Graphics.DrawLine(new Pen(Brushes.Black, 2), xC + 250, yC + 115, 700, yC + 115);
                yC += 150;

            }


            #endregion