For those who are getting confused whether why the answer to this is not 41
See what efficiency is $\eta=\frac{useful\,time}{Total\,Time}$
Now, this Total time is the time when the sender receives the ACK for the first packet sent and then it can start sending the packets in the next window(After receiving ACK only window will slide).
Now here they have only given you RTT=80ms.(So, this is the total time right from the moment sender started sending the first packet to the moment when the sender received it's first ack.
Now, Useful time=$N \times T_t$
Where N=Optimal Window Size for efficiency required $\eta$
and $T_t=$TIme required to transmit one 32 byte frame->$2ms$
We need optimal window size so $\eta=1$
By Definition given above
$1=\frac{N \times 2}{80} \Rightarrow N=40$