



 
Surender, One way to approach this problem is to write
and then find the units digit of 7^{21} and the units digit of 3^{21} using the technique in my response to Alex's question. Multiply the two units digits obtained to find the units digit of (7 × 3)^{21} = 7^{21} × 3^{21}. For this particular problem it is easier to deal with 7 × 3 = 21 directly. Your question is to find the units digit of (7 × 3)^{21} = 21^{21}.
Penny  


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