Sorry for all the colors, but I’m sure you can still get the general idea. In this example, I used numbers, but letters or symbols or whatever would also work.

Folded an old cardboard box, so the pins have somewhere to go, other than poking me.

Wrote in a whole bunch of numbers.

Added sewing pins in strategic places.

Cut some cardstock in strips, in matching colors.

Put the color strips on the matching color pins.

Answer is 7 – 3 – 8 because only those three areas have one number … all other areas have more than one number. Could be a lock combo or spell GCH or whatever else.