Least count and the error of a reading

A vernier callipers reads finer than a ruler because of its sliding scale. You will make the program find the least count of three verniers, read a bolt with each, and correct a reading for a zero error.

The idea. If $n$ vernier divisions fill $n - 1$ main divisions of $1\ \text{mm}$:

and the correct reading is the observed reading minus the zero error.

Your tasks.

1. Make least_count(n) return the least count in millimetres.
2. Make reading(main, line, n) return the reading in millimetres.
3. Correct the last reading for the zero error.