Answer:
720
Step-by-step explanation:
Knowing that he must place one of each type, the total number of marbles must be one of theses 6 cases:
3 blues, 1 green, 1 black
2 blues, 2 green, 1 black
2 blues, 1 green, 2 black
1 blues, 2 green, 2 black
1 blues, 1 green, 3 black
1 blues, 3 green, 1 black
At the same time, each case can be ordered in many ways, for example in the first option:
blue, blue, blue, green, black
blue, blue, blue, black, green
blue, blue, black, blue, green
et cetera
The total number of permutations for each case is 5! = 120
Taking into account the 6 cases, 6 x 120 = 720