reglade3.py

from PyQt5.QtWidgets import QGridLayout, QMainWindow, QWidget, QApplication, QLineEdit, QLabel
from PyQt5.QtGui import QIntValidator

class MainWindow(QMainWindow):
    def __init__(self, *args, **kwargs):       
        super(MainWindow, self).__init__(*args, **kwargs) 
        self.setWindowTitle("Regla de 3") 
        self.setupUi()

    def setupUi(self):
        # === Widget contenedor para MainWindow
        self.centralwidget = QWidget(self)
        self.setCentralWidget(self.centralwidget)
        # === Campos de texto
        self.editA1 = QLineEdit()
        self.editA2 = QLineEdit()
        self.editB1 = QLineEdit()
        self.editB2 = QLineEdit()
        # === Validadores numéricos para los textos
        self.editA1.setValidator(QIntValidator())
        self.editA2.setValidator(QIntValidator())
        self.editB1.setValidator(QIntValidator())
        self.editB2.setReadOnly(True)
        self.editB2.setStyleSheet("background-color: lightgray")
        # === Grid Layout para posicionamiento
        self.gridLayout = QGridLayout()
        self.gridLayout.addWidget(self.editA1, 0, 0) 
        self.gridLayout.addWidget(QLabel("es a"),  0, 1) 
        self.gridLayout.addWidget(self.editA2, 0, 2) 
        self.gridLayout.addWidget(QLabel("como"),  1, 1) 
        self.gridLayout.addWidget(self.editB1, 2, 0) 
        self.gridLayout.addWidget(QLabel("es a"),  2, 1) 
        self.gridLayout.addWidget(self.editB2, 2, 2) 
        # === Configuración del layout en el widget contenedor
        self.centralwidget.setLayout(self.gridLayout)
        # === Configuracin de las señales
        self.editA1.textChanged.connect(self.textChanged)
        self.editA2.textChanged.connect(self.textChanged)
        self.editB1.textChanged.connect(self.textChanged)
        self.editB2.textChanged.connect(self.textChanged)

    def textChanged(self):
        try:
            valorA1 = float(self.editA1.text())
            valorA2 = float(self.editA2.text())
            valorB1 = float(self.editB1.text())
            self.editB2.setText(str(valorA2 * valorB1 / valorA1))
        except Exception as e:
            print(e)
        
if __name__ == "__main__":    
    import sys
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec_())