At network one we have to divide the 1600 byte data.
At network 1:==> divide the packet into two part ( 1480 + 120 ) and add the 20 bytes of the head to each.
At network 2 :==> MTU = 480, here we have to divide 1480 into three parts + 20 byte of header to each. So, total packets we have ==== 4.
At network 3:==> No need to divide. the packes.
So, we got 4 packets.
1680 === 1600 (data) + 80 ( header for four packets).
1680.