This can be determined by doing some mapping:
The first member of the 5-member set can take 8 values
The second member of the 5-member set can take 7 values
The third member of the 5-member set can take 6 values
The fourth member of the 5-member set can take 5 values
The fifth member of the 5-member set can take 4 values
hence, the total number of injective functions possible = 8*7*6*5*4 = 6720
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