A speedometer of a truck reads $80{\text{ km h}}^{-1}$ with a systematic error of $\pm 3\%$ and a random error of $\pm 0.8{\text{ km h}}^{-1}$. What is the range of possible true speeds of the truck?

Step-by-step solution

1. Calculate systematic error in km/h: $3\%$ of $80=2.4{\text{ km h}}^{-1}$ (percent calculation).
2. Combine systematic and random errors in the worst case: subtract both from the reading for the lower bound: $80-2.4-0.8=76.8{\text{ km h}}^{-1}$.
3. Add both errors to the reading for the upper bound: $80+2.4+0.8=83.2{\text{ km h}}^{-1}$.
4. Thus the true speed lies between $76.8{\text{ km h}}^{-1}$ and $83.2{\text{ km h}}^{-1}$ (interval).

Concept tested: error propagation

Published 7 October 2026 · How this solution was prepared and checked