In a matched CMOS inverter in which Vtn = - Vtp = 0.5 V, VDD = 1.8 V, kn = kp = 600 uA/V^2 and fabricated in a 0.13 um technology, use the average current method to determine W/L ratios such that the propagation delay tp is smaller than or equal to 18 ps knowing that the load capacitance is equal to 20 fF.