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General number properties

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Source: — Problem Solving |
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by akhpad » Sun Apr 25, 2010 7:38 am
You need to know power cycle

power cycle of 2 is 2,4,8,6
power cycle of 3 is 3,9,7,1
power cycle of 5 is 5
power cycle of 7 is 7,9,3,1

I will take only UNIT digit
3^17X17^54X135^19X31^21X42^17?

= 3 * 9 * 5 * 2 = 270 (Unit digit is ZERO); it is the way to find unit digit only.
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by tpr-becky » Sun Apr 25, 2010 7:40 am
The units digit of any number is obtained by multiplying the units digits - you don't have to worry about all the other digits.

The 135^19 will always end in a 5 because a 5 to an exponent always ends in a 5. the 42^17 will end in a 2 becuase the units digits of 2 to an exponent have a pattern of 2, 4, 8, 6. if you multiply 5(2) you will always end up with a units digit and it won't matter what other numbers you multiply by.

This falls under the concept of when the math gets too hard there is a pattern - find it.
Becky
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by GmatTakerNo.1 » Sun Apr 25, 2010 7:45 am
Thank you guys.
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by harshavardhanc » Sun Apr 25, 2010 9:35 am
you just need a 5 and a 2 to decide the units digit, as it will be always be 0 if you find these two numbers in a product.

135 will give you a 5 and 42 will give you a two. That's it. therefore, the units digit of this messy product will be 0.
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