Can someone please explain to me ASAP??!!

Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates R1 = 2 Mbps, R2 = 2 Mbps, and R3 = 4 Mbps.

a. Assuming no other traffic in the network, what is the throughput for the file transfer?

b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?