Cyclicity in GMAT Remainder Questions
BY KARISHMA, VERITAS PREP
If n is a positive integer, what is the remainder when 3^(8n+3) + 2 is divided by 5?(A) 0
(B) 1
(C) 2
(D) 3
(E) 4
In this problem, we are looking for the remainder when the divisor is 5. We know from last week that if we get the last digit of the dividend, we will be able to find the remainder, so let’s focus on finding the units digit of 3^(8n + 3) + 2.
The units digit of 3 in a positive integer power has a cyclicity of: 3, 9, 7, 1
So the units digit of 3^(8n + 3) = 3^(4*2n + 3) will have 2n full cycles of 3, 9, 7, 1 and then a new cycle will start:
3, 9, 7, 1
3, 9, 7, 1
3, 9, 7, 1
…
3, 9, 7, 1
3, 9, 7
Since the exponent a remainder of 3, the new cycle ends at 3, 9, 7. Therefore, the units digit of 3^(8n + 3) is 7. When you add another 2 to this expression, the units digit becomes 7+2 = 9.
This means the units digit of 3^(8n+3) + 2 is 9. When we divide this by 5, the remainder will be 4, therefore, our
answer is E. This question is discussed
HERE.
Not so bad; let’s try a data sufficiency problem:
If k is a positive integer, what is the remainder when 2^k is divided by 10?Statement 1: k is divisible by 10
Statement 2: k is divisible by 4
With this problem, we know that the remainder of a division by 10 can be easily obtained by getting the units digit of the number. Let’s try to find the units digit of 2^k.
The cyclicity of 2 is: 2, 4, 8, 6. Depending on the value of k is, the units digit of 2^k will change:
If k is a multiple of 4, it will end after one cycle and hence the units digit will be 6.
If k is 1 more than a multiple of 4, it will start a new cycle and the units digit of 2^k will be 2.
If k is 2 more than a multiple of 4, it will be second digit of a new cycle, and the units digit of 2^k will be 4.
If k is 3 more than a multiple of 4, it will be the third digit of a new cycle and the units digit of 2^k will be 8.
If k is 4 more than a multiple of 4, it will again be a multiple of 4 and will end a cycle. The units digit of 2^k will be 6 in this case.
and so on…
So what we really need to find out is whether k is a multiple of 4, one more than a multiple of 4, two more than a multiple of 4, or three more than a multiple of 4.
Statement 1: k is divisible by 10
With this statement, k could be 10 or 20 or 30 etc. In some cases, such as when k is 10 or 30, k will be two more than a multiple of 4. In other cases, such as when k is 20 or 40, k will be a multiple of 4. So for different values of k, the units digit will be different and hence the remainder on division by 10 will take multiple values. This statement alone is not sufficient.
Statement 2: k is divisible by 4
This statement tells you directly that k is divisible by 4. This means that the last digit of 2^k is 6, so when divided by 10, it will give a remainder of 6. This statement alone is sufficient. therefore our
answer is B. This question is discussed
HERE.
Now, to cap it all off, we will look at one final question. It is debatable whether it is within the scope of the GMAT but it is based on the same concepts and is a great exercise for intellectual purposes. You are free to ignore it if you are short on time or would not like to go an iota beyond the scope of the GMAT:
What is the remainder of (3^7^11) divided by 5?(A) 0
(B) 1
(C) 2
(D) 3
(E) 4
For this problem, we need the remainder of a division by 5, so our first step is to get the units digit of 3^7^{11}. Now this is the tricky part – it is 3 to the power of 7, which itself is to the power of 11. Let’s simplify this a bit; we need to find the units digit of 3^a such that a = 7^{11}.
We know that 3 has a cyclicity of 3, 9, 7, 1. Therefore (similar to our last problem) to get the units digit of 3^a, we need to find whether a is a multiple of 4, one more than a multiple of 4, two more than a multiple of 4 or three more than a multiple of 4.
We need a to equal 7^{11}, so first we need to find the remainder when a is divided by 4; i.e. when 7^{11} is divided by 4.
For this, we need to use the binomial theorem we learned earlier in this post (or we can use the method of “pattern recognition”):
The remainder of 7^{11} divided by 4
= The remainder of (4 + 3)^{11} divided by 4
= The remainder of 3^{11} divided by 4
= The remainder of 3*3^{10} divided by 4
= The remainder of 3*9^5 divided by 4
= The remainder of 3*(8+1)^5 divided by 4
= The remainder of 3*1^5 divided by 4
= The remainder of 3 divided by 4, which itself = 3
So when 7^{11} is divided by 4, the remainder is 3. This means 7^{11} is 3 more than a multiple of 4; i.e. a is 3 more than a multiple of 4.
Now we go back to 3^a. We found that a is 3 more than a multiple of 4. So there will be full cycles (we don’t need to know the exact number of cycles) and then a new cycle with start with three digits remaining:
3, 9, 7, 1
3, 9, 7, 1
3, 9, 7, 1
…
3, 9, 7, 1
3, 9, 7
With this pattern, we see the last digit of 3^7^11 is 7. When this 7 is divided by 5, remainder will be 2 – therefore, our
answer is C. This question is discussed
HERE.