Exercise 1 solution

Four thin lines between 150 and 200 at a 10 m interval gives lines at
160, 170, 180 and 190 (five steps of 10 m make 50 m).

- Second thin line above 150: 170 m.
- Line directly above 200: 210 m.
- Third line above 200: 230 m.
- Path from 170 m to 230 m: the lines are 170, 180, 190, 200, 210, 220, 230,
  so it crosses 7 lines counting both ends (6 if you do not count the line
  you start on).

Method: start at a labelled line, then add or subtract the interval once
for every line you pass.