A simple transistor buffer is needed to drive a length coaxial cable containing a capacitance of 100pF per meter length. The whole steady-state power dissipation of the buffer should not exceed 250mW and the propagation delay of a pulse transmitted along the cable should not exceed 100ns. There buffer is to be driven through a TTL logic gate. When a transistor is available that has a minimum βF of 50, calculate the maximum length of cable which can be driven.