GmatPrep I--x^4 + y^4 = 100

prachich1987 wrote:If x^4 + y^4 = 100, the greatest possible value of x is between

a. 0 and 3
b. 3 and 6
c. 6 and 9
d. 9 and 12
e. 12 and 15

(x^4 + y^4) = 100 => x^4 = (100 - y^4)

Note that when x is maximum, x^4 is also maximum. Now to maximize x we have to maximize (100 - y^4), which is equivalent to minimization of y^4. Now minimum value of y^4 is 0. Therefore maximum value of (100 - y^4) is 100.

Thus, maximum value of x^4 is 100.
Therefore, maximum value of x = 4-th root of 100 = √10 ≈ 3.(something)

The correct answer is B.

prachich1987 wrote:Thanks Anurag.
Even I had marked B.
But the answer given in the document is A.It must be wrong.

yep

i noticed that you titled this thread "gmatprep I" -- did you actually see this problem on gmatprep, with this solution, with your own two eyes? or did you get it from some kind of archive?

prachich1987 wrote:

lunarpower wrote:

prachich1987 wrote:Thanks Anurag.
Even I had marked B.
But the answer given in the document is A.It must be wrong.

yep

i noticed that you titled this thread "gmatprep I" -- did you actually see this problem on gmatprep, with this solution, with your own two eyes? or did you get it from some kind of archive?

This is from the document "198 Level 700+ questions from GMAT Prep I which I downloaded from BTG.

ah. ok.

i downloaded it and wrote a post about it here:
https://www.beatthegmat.com/198-level-70 ... tml#327982

there are a couple of other sketchy problems in there -- for instance, #74 says that two INTEGERS x and y have a sum of 72, but statement (1) says that x = y + 1. this is impossible; if x = y + 1 and x+y = 72, then you get x = 36.5 and y = 35.5, contradicting the statement that they are integers.
so, overall, that's a decent source, but (a) be careful, and (b) they aren't all "top difficulty" problems (not that this matters anyway -- it doesn't).
in fact, any claim by anyone who purports to know the exact difficulty levels of official GMAC problems will always be a lie; GMAC has never published the adaptive performance curves of any of its problems, nor does it ever plan to do so.