The next window will be sent after receiving the ACK of the first packet of the window.
one packet transmission time = 10 micro sec
For transmission time 50 micro sec required for the whole window
After transmission, the first packet ACK reaches the sender is Tt+ 2Tp = 510 micro sec.
The sender is busy up to 50 micro sec.
So it start sending the new window = 510 - 50 = 460 micro sec