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The units digit of the square of the number 12345 is 1.

The units digit of 12345 is 5.

We know that if the units digit of a number is 5, the units digit of the square of that number will also be 5. 

(12345)²  = 12345 * 12345 = 152,399,025

Thus, the unit digit of the square of 12345 is also 5.