# Thread: Problem dealing with some Number Theory "multiple of some #"

1. ## Problem dealing with some Number Theory "multiple of some #"

They say, 100 is a multiple of 4, so that's why the units digit of 3100 is the same as that of 34 However, 100 is also a multiple of 5. So why did they pick 4?

Please help explain this--as I am not hot in number theory

If n is an integer with units digit 3,
what is the units digit of n 100 ?

Since only the units digit of n matters, you can start by exploring what happens when n equals 3. Raise 3 to powers and observe the units digits of the results.
31 = 3
32 = 9
33 = 27
34 = 81
35 = 243

units digit:
3
9
7
1
3

Notice that the units digit of 35 is again 3, so from here on the sequence will repeat itself and continue to cycle through the four values 3, 9, 7, 1. Since 100 is a multiple of 4, the units digit of 3100 is the same as that of 34

2. Originally Posted by eri
They say, 100 is a multiple of 4, so that's why the units digit of 3100 is the same as that of 34 However, 100 is also a multiple of 5. So why did they pick 4?

Please help explain this--as I am not hot in number theory

If n is an integer with units digit 3,
what is the units digit of n 100 ?

Since only the units digit of n matters, you can start by exploring what happens when n equals 3. Raise 3 to powers and observe the units digits of the results.
31 = 3
32 = 9
33 = 27
34 = 81
35 = 243

units digit:
3
9
7
1
3

Notice that the units digit of 35 is again 3, so from here on the sequence will repeat itself and continue to cycle through the four values 3, 9, 7, 1. Since 100 is a multiple of 4, the units digit of 3100 is the same as that of 34
at first, i can't understand what you posted.. well, i will edit this for you..

Originally Posted by eri
They say, 100 is a multiple of 4, so that's why the units digit of $3^{100}$ is the same as that of $3^4$ However, 100 is also a multiple of 5. So why did they pick 4?

Please help explain this--as I am not hot in number theory

If n is an integer with units digit 3,
what is the units digit of
$n^{100}$ ?
Since only the units digit of n matters, you can start by exploring what happens when n equals 3. Raise 3 to powers and observe the units digits of the results.
$3^1 = 3$
$3^2 = 9$
$3^3 = 27$
$3^4 = 81$
$3^5 = 243$
units digit:
3
9
7
1
3

Notice that the units digit of $3^5$ is again 3, so from here on the sequence will repeat itself and continue to cycle through the four values 3, 9, 7, 1. Since 100 is a multiple of 4, the units digit of $3^{100}$ is the same as that of $3^4$^
well, the explanation was given to you..
note that the unit digit repeats every after 4 powers..
obeserv also that
1= 4(0)+1
2=4(0)+2
3=4(0)+3
4=4(1)+0
5=4(1)+1

and with powers 1,5,9,... etc, they will have unit digit as 3;
with powers 2,6,10,... etc, they will have unit digit as 9;
with powers 3,7,11,... etc, they will have unit digit as 7;
with powers 4,8,12,... etc, they will have unit digit as 1;

now, 100=4(25)+0.. and notice that the similarity i has with 4=4(1)+0 is the thing you added to the multiple of 4..

why pick 4 and why not pick 5? the unit digits repeat every after 4 powers..

you may compute for $3^{99}$.. 99 = 4(24)+ 3.. because of this, the unit digit of $3^{99}$ is the same as the unit digit of $3^3$ which is 7..