Solution: This is the Stirling number of the second kind $ S(6, 3) $, calculated as $ \frac13! \left( 3^6 - 3 \cdot 2^6 + 3 \cdot 1^6 \right) = \frac729 - 192 + 36 = \frac5406 = 90 $. - Groen Casting