Thank you for the opportunity to help you with your question!
Notice how the data is close to the shape of a normal curve? We see the mean in this example is 160 because it falls right in the middle of the curve at the highest peak. We then see the position of the first standard deviation to the right and left of the mean represented by 180 and 140 respectively.
The value of the standard deviation is the difference between the mean and it's first standard deviation position on the normal curve, which in this case is 180 - 160 = 20.
Please let me know if you need any clarification. I'm always happy to answer your questions.