What is the instrumental uncertainty of the clock and the odometer?

Every measuring instrument has an inherent uncertainty that is determined by the precision of the instrument.

The instrumental uncertainty is 50%

Every instrument has some uncertainty. No instrument is accurate. It depends on the least count of the instrument. The accuracy of a measurement is given by its uncertainty. The uncertainty is generally calculated as 50% of the least count of the instrument. Let us say, a clock can measure precisely till 1 s. it means it has a least count of 1 s. Then uncertainty of this clock would be 50% of 1 s = 0.5 s. Odometer is an instrument in a vehicle which measures and displays the total distance traveled. Let us say the odometer has a least count of 1 km. Then, its uncertainty would be = 50% of 1 km=0.5 km.