minimum MTU given is 480.. 20 bytes for header+ 460 bytes of payload
since 460 is not divisible by 8.. so we would take 456 bytes... now for 1600 datagram,1600 bytes have to be transferred in packet of size 456..
no of packets= 1600/456=3.5=4 packet.. for 4 packets 20 bytes included in each packet.. so 1680 bytes...