Charts represent the “reduced earth”.
The chart scale represents the relation between the length represented on the chart compared with the real distance on planet earth.
Chart Scale = Chart length / Earth distance
On a Mercator chart, scale expands away from the Equator.
The scale expansion is proportional to the secant of the latitude.
Questions about scale in a Mercator chart, come in 1 of 4 basic types:

Given scale at the Equator, find Mercator scale at some other latitude.

Given Mercator scale at some latitude, find scale at the Equator.

Given Mercator scale at some latitude, find Mercator scale at some other latitude.

Given a fixed chart distance between meridians, find Mercator scale at a specific latitude.
The equation to remember to solve this questions is the following one:
Scale denominator A x cos (Latitude B) = Scale denominator B x cos (Latitude A)
3 000 000 x cos (0º) = Scale denominator Equator x cos (60º)
Scale denominator Equator = 6 000 000
Scale at the Equator = 1 / 6000 000
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