With slow start, the first RTT sends out 1 segment (or 2KB), the 2nd RTT sends out 2 segments (or 4KB), the 3rd 4 segments (or 8KB), the 4th 8 segments (or 16KB). The 5th RTT would have sent out 16 segments (or 32KB), however, it'll exceed the receiver's window. Therefore, the amont of time it takes BEFORE the 5th RTT (or full window, that is, 24KB) is 4*10=40msec.