1. ## 10 lucky digits

find the 10 digits of 3^2007.

how many 4-digit numbers whose digits are all odd are multiples of 11?

2. Originally Posted by samsum
find the 10 digits of 3^2007.
I will use Congruences and Euler's Generalization of Fermat's Little Theorem..

We note that,
$\gcd(3,100)=1$

And, $\phi(100)=100\left(1-\frac{1}{2} \right)\left(1-\frac{1}{5}\right)=100(1/2)(4/5)=40$

Thus,
$3^{40}\equiv 1(\mbox{mod }100)$

Bring both sides to the power of 50 thus,
$3^{2000}\equiv 1(\mbox{mod }100)$
Note that,
$3^7\equiv 87(\mbox{mod }100)$
Thus, their multiplication yields,
$3^{2007}\equiv 87(\mbox{mod }100)$

Since it is congruent to 100 it shows the last two digits. The second to last digits it 8.

3. Originally Posted by samsum
how many 4-digit numbers whose digits are all odd are multiples of 11?
You have a 4 digit number,
$ABCD$ with $A\not = 0$
und,
$0\leq A,B,C,D\leq 9$

The necessary and suffient condition for the divisibility of 11 to exist we need that the Alternating sum is a divisible by 11. Thus,
$D-C+B-A=0 \mbox{ or } 11 \mbox{ or } -11$
Thus,
$D+B=A+C$
$D+B=A+C+11$
$11+D+B=A+C$
Basic trial and error will produce all solutions.

4. Originally Posted by samsum
how many 4-digit numbers whose digits are all odd are multiples of 11?
The smallest 4 digit number divisible by 11 is 91*11, and the largest
4 digit number divisible by 11 is 909*11. Therefore there are 819 such
numbers divisible by 11. Of these (819-1)/2 are even and the rest are odd
so there are 410 odd numbers 4 digit divisible by 11.

RonL