Exercise 2 solution

Rule used (variation is west of grid north, as in the exercise):
  Grid to magnetic: ADD the variation.
  Magnetic to grid: SUBTRACT the variation.
A memory aid: "Grid to Mag, add" (and the reverse for the other direction).

(a) Grid bearing 90 degrees, variation 2 degrees west:
    magnetic = 90 + 2 = 92 degrees

(b) Magnetic bearing 90 degrees, variation 2 degrees west:
    grid = 90 - 2 = 88 degrees

(c) Grid bearing 1 degree, variation 2 degrees west:
    magnetic = 1 + 2 = 3 degrees

(d) Magnetic bearing 1 degree, variation 2 degrees west:
    grid = 1 - 2 = -1, add 360 = 359 degrees

Always wrap the result back into the range 0 to 360: add 360 if it goes
below 0, subtract 360 if it goes above 360.

If the variation were east, the rules swap: subtract going from grid to
magnetic, add going from magnetic to grid.