Perimeter = 2(Length + Breadth). 40 = 2(12 + B) => 20 = 12 + B => B = 8 cm.
परिमाप = 2(लंबाई + चौड़ाई)। 40 = 2(12 + B) => 20 = 12 + B => B = 8 सेमी।