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Power 2: units digit 9. Multiply by 49 again to get power 4: units digit 1. So every 4th power gives units digit 1. So 16th power has units digit 1, so the previous power, the 15th must have units digit 3.
The last digit of a number raised to a power can be determined by finding a pattern in the units digits of the number's powers. For 2 raised to the power of 1997, the units digit will follow a pattern of 2, 4, 8, 6. Since 1997 is one less than a multiple of 4, the last digit will be 8.
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When multiplying numbers with the same units digit, the units digit of the product is determined by the units digit of the base number raised to the power of the number of times it is being multiplied. In this case, since 7 is being multiplied 100 times, the units digit of the product will be the same as the units digit of 7^100. The units digit of 7^100 can be found by looking for a pattern in the units digits of powers of 7: 7^1 = 7, 7^2 = 49, 7^3 = 343, 7^4 = 2401, and so on. The pattern repeats every 4 powers, so the units digit of 7^100 will be the same as 7^4, which is 1. Therefore, the units digit of the product when one hundred 7's are multiplied is 1.
To find the units digit of a number raised to a power, we can look for patterns in the units digits of the powers of that number. For 2, the units digits of the powers cycle in a pattern: 2, 4, 8, 6. Since 2011 is 3 more than a multiple of 4 (2011 = 4 * 502 + 3), the units digit of 2 to the power of 2011 will be the fourth number in the cycle, which is 6.