Renting System

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.Windows.Forms;


namespace StudioRentingSystem
{
    public partial class Renting_System : Form
    {
        public Renting_System()
        {
            InitializeComponent();
        }

        private void btnCalculate_Click(object sender, EventArgs e)
        {
            //Variables
            double totalCustomerOwe = 0;
            double noMinUsed = 0;
            double charged = 0;
            const int RATE_PER_HOUR = 350;

            //Covert string to number
            noMinUsed = Convert.ToInt32(noMinUsedField.Text);

            //Calculation
            charged = RATE_PER_HOUR / 60;
            double minRate = Math.Round(charged);
            totalCustomerOwe = minRate * noMinUsed;

            //Result Display
            //Side Summary display
            lblStudioNameResult.Text = studioNameField.Text;
            lblStudioNameResult.Text = studioNameField.Text;
            lblFixRatePerHour.Text = lblFixRatePerHour.Text + RATE_PER_HOUR.ToString("N2");
            lblRatePerMinutes.Text = lblRatePerMinutes.Text + minRate.ToString("N2");

            //Total Summary Display
            lblStudioName.Text = lblStudioName.Text + studioNameField.Text;
            lblNoMinUsed.Text = lblNoMinUsed.Text + noMinUsed.ToString();
            lblRatePerHour.Text = lblRatePerHour.Text + "PHP " + RATE_PER_HOUR.ToString("N2");
            lblRatePerMin.Text = lblRatePerMin.Text + "PHP " + minRate.ToString("N2");
            lblTotalCustomerOwe.Text = lblTotalCustomerOwe.Text + "PHP " + totalCustomerOwe.ToString("N2");
        }

        private void btnClear_Click(object sender, EventArgs e)
        {
            studioNameField.Text = "";
            noMinUsedField.Text = "";
            lblStudioName.Text = "STUDIO NAME: ";
            lblNoMinUsed.Text = "NO. MINUTES USED: ";
            lblRatePerHour.Text = "RATE PER HOUR: ";
            lblRatePerMin.Text = "RATE PER MIN: ";
            lblTotalCustomerOwe.Text = "TOTAL CUSTOMER OWE: ";

            lblStudioNameResult.Text = "STUDIO NAME";
            lblFixRatePerHour.Text = "FIXED RATE/HOUR: ";
            lblRatePerMinutes.Text = "RATE/MIN: ";
        }

        private void btnExit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void noMinUsedField_KeyPress(object sender, KeyPressEventArgs e)
        {
            // allows only numbers
            if (!char.IsNumber(e.KeyChar) & (Keys)e.KeyChar != Keys.Back & e.KeyChar != '.')
            {
                e.Handled = true;
                MessageBox.Show("Input should not be letters/characters!");
            }
        }
    }
}