dave13 wrote:
PKN wrote:
Bunuel wrote:
What is the value of x?
(1) x^4 = 2,401
(2) x^5 = 16,807
Question stem:- x=?
St1:- \(x^4 = 2,401\)
Here we have 4 values of x, viz, 7, -7, 7i, and -7i.
Note:- GMAT accepts only real solutions/values. So, discard 7i and -7i.
Hence, x=7,-7
No unique value of x.
Insufficient.
St2:- \(x^5 = 16,807\)
Or, \(x= 16807^{\frac{1}{5}}\)
Or, \(x= (7^5)^{\frac{1}{5}}\)
Or, x=7 (\(\sqrt[n]{a^n}=a\))
Unique value of x.
Hence, sufficient.
Ans. (B)
PKNhow did you calculate \((7^5)\) in second statement?
Thanks
Hi
dave13,
Nice question indeed!!?
How did I choose \(7^5\)?
GMAT maintains a pattern,i.e, 99% of the times,the test makers set DS questions(value questions with correct option D, such as what is the value of x? ) in such a way that each statement yields the same unique value.
After we are done with st1(we found x to be 7 or -7), on the first sight, our priority is to check statement 2 whether it yields x=7 as a possible value.
Already we have calculated till \(7^4\), hope it is not a cumbersome task to multiply 7 again with \(7^4\).( A quick note in mind:- unit digit of \(7^4\) and 7,when multiplied yield 7 (☺️16807 has units digit 7)).
Hope my thought process helps .
P.S.- An example for you.
Q. What is the value of (a-b)?
1) \((a-b)^4=16\)
2) \((a-b)^5=-32\)
Again ,how did I know 2401=\(7^4\)? Let me answer you before you ask me really. Please apply divisibility rule of 2,3,4,5,6,7 as quickly as possible. I hope you know the divisibility tests.
Another method:- ( without doing anything)
1. You know GMAT lives in real world . So any root(even root or odd root) will always yield a real value.
2.Even roots yield both positive values and negative values when the number is positive. For negative numbers,even root is imaginary. (Here the number is positive,so,from st1 we have two values of x.hence,insufficient)
3. Odd roots yield positive values when the number is positive.Odd roots yield negative values when the number is negative. (Here the number is positive ,so the odd root will yield a positive unique value.Hence sufficient)
If you know the concepts and reasoning, it will hardly take 10s to conclude the answers , subsequently saves 1 minute of your precious time in real exam.
_________________
Regards,
PKN
Rise above the storm, you will find the sunshine