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19 Jun 2009, 07:16
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A certain sequence starts with the number 4. Every sub-sequent term is twice the number 4. Every sub-sequent term is twice the previous one, What is the units digit of the 79 term?
A)8
B)6
C)4
D)2
E)0
I tried to applied the following method
4 firstterm
8 second
16 third
32 fourth
64 fifth
...8 sixth
...6 seventh
...2 eighth
...4 ninth
...8 Tenth
So the Tenth term has a unit digit of 8 I thought that the 80th term would also have a term of 8 so one term less would have 4 the unit-s digit but the answer is D) so what am I doing wrong?
A)8
B)6
C)4
D)2
E)0
I tried to applied the following method
4 firstterm
8 second
16 third
32 fourth
64 fifth
...8 sixth
...6 seventh
...2 eighth
...4 ninth
...8 Tenth
So the Tenth term has a unit digit of 8 I thought that the 80th term would also have a term of 8 so one term less would have 4 the unit-s digit but the answer is D) so what am I doing wrong?
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19 Jun 2009, 08:06
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TheRob
I think you are going in the right direction. Last digit of 1st 4 numbers would be 4 (try 4*8*16*32).
Similarly, for product of each subsequent set of 4 numbers, last digit would be 4.
So, this means we have to find last digit of
..4*..4*..4.....(19 times)* ..4 * ..8 *...6
Now, for each 4*4*4, last digit would be 4
This reduces to
..4*..4*..4...(6 times) *4 *..4 * ..8 *...6
=(..4*..4) *..4 *..4 * ..8 *...6
=..4*..4 * ..8 *...6
=..8
So, last digit should be 8, answer A
