What determines the precision of a calculated result based on a measurements?

Answer 1

the precision of the least precise measuement

Answer 2

In olden times, units of measure were often based on something arbitrary, such as a feature of the reigning monarch (the length of a foot was the length of his foot; that of a yard was the distance from his nose to the tip of his longest finger). These days, units of measurements have been standardized: for example, the meter is the distance light travels in 1/299,792,458 of a second. Lasers, GPS devices, and other high-tech tools allow us to measure distance, time, weight, etc. to a level of precision only dreamed about even half a century ago.