The height at which the atmosphere ceases to exist is about 80km. If the atmospheric pressure on the ground is 760 mmHg, the pressure at a height of 20km above ground level is

Pressure is directly proportional to height. The height of the atmosphere above someone $20 km$ above ground level is $80 km - 20km = 60 km$. Therefore, the atmospheric pressure at that altitude will be $\frac{60}{80} \times 760 = 570 mmHg$