Final answer:
The '20s' in '20/20 vision' refer to the distance at which a person with normal acuity can read letters that subtend a visual angle of 5 minutes of an arc.
Step-by-step explanation:
The term '20/20 vision' refers to the measurement of visual acuity. The first '20' represents the distance at which a person with normal acuity can read letters that subtend a visual angle of 5 minutes of an arc. The second '20' represents the distance at which someone with 20/20 vision can read those same letters. For example, if you have 20/60 vision, it means that you can read letters that a person with normal acuity can read from 60 feet away, but you have to be 20 feet away to read those same letters. In this context, the unit of measurement for the '20s' is feet.