4.
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The last digit is 4.
It is the last digit of 34= 81. Therefore it is 1.It is the last digit of 34= 81. Therefore it is 1.It is the last digit of 34= 81. Therefore it is 1.It is the last digit of 34= 81. Therefore it is 1.
6. All even powers of 4 end in 6
I assume you mean the last digit. The idea is to find a pattern: 8 to the power 1 ends with 8 8 to the power 2 ends with 4 8 to the power 3 ends with 2 (to figure this out, I just multiplied 4 x 8) 8 to the power 4 ends with 6 (once again, multiply the last digit of the previous power x 8) 8 to the power 5 ends with 8 8 to the power 6 ends with 4 ... As you can see, it all repeats, with a period of 4. Therefore, any exponent that is a multiple of 4 will end with 6. Find the nearest multiple of 4 that is smaller than 2015, then continue calculating - always with the last digit.
The digit with which a multiple of 4 ends depends on the last digit of the other factor. If the last digit is a zero, the product ends with zero; if the last digit is a 1, the product ends with 4; etc. The only options for the last digit of the product are 0, 2, 4, 6, 8.