Question 76
A *Colombian Square* is a $6 \times 6$ square which is subdivided into $36$ unit squares, each of which is coloured either Yellow, Blue or Red according to the following rules:
a. No row or column may contain more than two unit squares of the same colour.
b. In any set of four unit squares obtained by intersecting two rows with two columns, no colour is to occur exactly three times.
How many different Colombian Squares are there?
a. No row or column may contain more than two unit squares of the same colour.
b. In any set of four unit squares obtained by intersecting two rows with two columns, no colour is to occur exactly three times.
How many different Colombian Squares are there?