A linear calibration slope must come from checked standards. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two standards define the training slope
Standard 2 gives signal 4; standard 6 gives signal 12.
m=2
The inverse result is fixed by the same slope
The slope is 2. Unknown signal 10 gives calibrated value 5.