The reason the absolute error is 0.1*10^3 is because you are using a ruler to measure out 1m to then count how many lines lie in that range, and in doing that measurement of 1m you are actually taking to readings, a reading of where the zero mark is and a reading of where the 1m mark is so you’re absolute error of each reading is 0.5*10^3 lines so overall absolute error is 1*10^3