As i said, there is no standard definition of RTT , it depends upon question.
like RTT = 2 * Tp ,( by not including Transmission time ) also possible. For long distance network like Satellite , we ignore the Transmission time as we don't know how long that receiver is.
And also in both questions there is simultaneous data transmission and sliding window is used .
Actual meaning of RTT is : RTT is the time between a frame is sent and its ACK is received.
Now depends upon question , we use formula.
This is actual how RTT value is calculated :
RTT = Propagation time for frame + Transmission time for frame + Propagation time for ACK + Transmission time for ACK
now generally we don't consider Transmission time for ack as it is piggybacked . thts why RTT formula becomes 2 * Tp + Tt
but in this question, https://gateoverflow.in/43470/gate2009-58 we have to add that Transmission time ack just because in question it says
After sending 2I frames , what is the minimum time the sender will have to wait before starting transmission of the next frame?
-- means after the transmission time of those 2I frames , how much time it should wait to start transmitting again , which means we need that time until ACK arrives. So, we just have to find the time when ACK arrives and that's why RTT value counts Transmission time for ack here in this particular question.
Hope , somehow i clear your doubt ..this is good question from your side. And i only see in this 2009 question RTT is counted like this way ..almost everywhere we use RTT formula straightforward way like RTT = 2 * Tp + Tt ...