Sliding Window Protocol, waiting Time calculation
You have not calculated the $R_{TT}$ you just used the Propagation Time that a frame needs to travel one way of the duplex link. 

This is how you calculate the $R_{TT}$:

$R_{TT} =$ [Propagation time for frame] + [Transmission time for frame] + [Propagation time for ACK] + [Transmission time for ACK]

Since ACK is piggybacked we assume that its frame size will also be 1000bits, so the $R_{TT}$ is:

$R_{TT} = 25ms + 1ms + 25ms + 1ms = 52ms $

Therefore waiting time is: $52ms - 32ms = 20ms$