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Re: To find the units digit of a large number? [#permalink]
03 Jul 2013, 22:10

1

This post received KUDOS

Expert's post

fozzzy wrote:

So if we are given a question what is the units digit of 777^{777}

we find the pattern for 7 (7,9,3,1)

then we divide \frac{777}{4} and the remainder is 1 so the units digit is 7^1 which is 7?

Is this correct?

Yes.

The units digit of 777^777 = the units digit of 7^777.

7^1 has the units digit of 7; 7^2 has the units digit of 9; 7^3 has the units digit of 3; 7^4 has the units digit of 1. 7^5 has the units digit of 7 AGAIN.

The units digit repeats in blocks of 4: {7, 9, 3, 1}...

The remainder of 777/4 is 1, thus the units digit would be the first number from the pattern, so 7.

Re: To find the units digit of a large number? [#permalink]
03 Jul 2013, 22:23

One final question here is another example

If we have the find the units digit of 344^{328}

4^1 is 4 4^2 is 16 4^3 is 4

so the repeating block over here {4,6}

In this case the remainder is 0 so the units digit of this expression is 6?

so if there are 4 repeating blocks and the remainder is 0 we raise it to the 4th power ( some examples would be 3,7 etc) in this current example its the 2nd power? _________________

Re: To find the units digit of a large number? [#permalink]
03 Jul 2013, 22:26

1

This post received KUDOS

Expert's post

fozzzy wrote:

One final question here is another example

If we have the find the units digit of 344^{328}

4^1 is 4 4^2 is 16 4^3 is 4

so the repeating block over here {4,6}

In this case the remainder is 0 so the units digit of this expression is 6?

so if there are 4 repeating blocks and the remainder is 0 we raise it to the 4th power ( some examples would be 3,7 etc) in this case its the 2nd power?

Yes, if the remainder is 0, then take the last digit from the block. _________________

Re: To find the units digit of a large number? [#permalink]
07 Jul 2013, 20:00

1

This post received KUDOS

fozzy,

you are correct in both cases. here is how i like to think about it: 7^{777} example: If we divide 777 by 4, the quotient is 194 and the remainder is 1. This means that we will have 194 of {7, 9, 3, 1} repeating blocks, and we will have 1 more term left, and the units digit of 7^{777} will be 7, the first term in the repeating block.

When we look at the case of 344^{328}, the units digit of powers of 4 cycle as {4, 6}, when we divide 328 by 2, the remainder is 0, this means that there will be exactly 164 blocks consisting of {4,6} without any remainder and the units digit of 344^{328} will be 6.