The initial speed, u of plane in terms of velocity of sound which may be taken as U u=142/331=0.429*U It crosses the sound barrier after says t seconds then we have 331-142=23.1*t or t is given 8.18 s exactly t=9/11s. After 18 seconds the plane will traveling with velocity V V=142+18*23.1=557.8 m/s==1.685*U
