Selector selects one device at a time, Multiplexer for getting output from one or more than one device.
For Selector I/O transfer rate = maximum of (3 HDD and 1 magnetic tape) = max(800,400) =800KBps
For Multiplexer I/O transfer rate = 2*1 + 4*2 = 2+8=10KBps
Total maximum aggregate I/O transfer rate = 3*800+2*10 =2400+20=2420KBps