I think, considering the base file is wrong, please correct me if I am wrong. In a non-persistent connection for each object, we need 3 RTTs (1 RTT for connection establishment,1RTT for object transfer by req-res +1 RTT for a connection termination so a total= 3 RTT ) so in this question 3 images =3*3RTT=9RTT=>18 messages, but in this question, it is asking several messages for receiving 3 messages so we will eliminate last connection termination so total =18-2=16 messages.