Answer:
- In miles per hour: 82 mph
Explanation:
The correct number of digits to report a measure include every certain digit plus one uncertain digit.
The speedometer has marks for:
- every 2 km per hour, km/h. Because each small tick mark on the outer circle represents 2 mph.
- every 10 miles per hour mph. Because each small tick mark on the inner scale represents 10 mph.
As a rule you can appreciate up to half way between two small tick marks.
So, for km/h you can appreciate half of 2 km/h, i.e. 1 km/h.
The needle is marking between 120 km/h and 122 km/h, which means that the correct measure is 121 km, where the two leftmost digits are certain and the ones digit is uncertain.
For mph, you can appreciate half of 10 mph, i.e. 5 mph. A measure between 80 and 90 mph should be read as 85 mph.
Furthermore, in this last case of mph, the space between two consecutive marks is broad and equal to 10 mph but also the needle is wide, so it is difficult to appreciate less than 5 mph.
The correct measure must have two significant digits: one certain and one uncertain. So, 82 mph is as correct as 85 mph, since the 8 is certain and the next number (either 2 or 5) is uncertain.
