2018 AMC 8 Problems/Problem 25


How many perfect cubes lie between $2^8+1$ and $2^{18}+1$, inclusive?

$\textbf{(A) }4\qquad\textbf{(B) }9\qquad\textbf{(C) }10\qquad\textbf{(D) }57\qquad \textbf{(E) }58$

Solution 1

We compute $2^8+1=257$. We're all familiar with what $6^3$ is, namely $216$, which is too small. The smallest cube greater than it is $7^3=343$. $2^{18}+1$ is too large to calculate, but we notice that $2^{18}=(2^6)^3=64^3$, which therefore clearly will be the largest cube less than $2^{18}+1$. So, the required number of cubes is $64-7+1= \boxed{\textbf{(E) }58}$.

Solution 2 (Brute force) (works only if you have enough time for calculations)(Do not use this in AMC8)

First, $2^8+1=257$. Then, $2^{18}+1=262145$. Now, we can see how many perfect cubes are between these two parameters. By guessing and checking, we find that it starts from $7$ and ends with $64$. Now, by counting how many numbers are between these, we find the answer to be $\boxed{\textbf{(E) }58}$.

Solution 3 (Guessing)

First, we realize that question writers like to trick us. We know that most people will be calculating the lowest and highest number whose cubes are within the range. The answer will be the highest number $-$ the lowest number $+ 1$. People will forget the $+1$ so the only possibilities are C and E. We can clearly see that C is too small so our answer is $\boxed{\textbf{(E) }58}$.

Solution 4 (If you do not notice that $2^{18}=(2^6)^3=64^3$)

There is not so much guessing and checking after we find that it starts from $7$ because $7^3=343$, which is over $2^8+1=257$. We can start guessing with the 10, answer C, as it is the middle value. Adding 10 to 7 gives us 17 and $17^3 = 4,913$, which is a bit low. So, we move "up" to 57, answer D. Adding 57 to 7 gives us 64 and $64^3 = 262,144$, which is perfect for the $2^{18}+1=262,145$. But we are not done. Since it is inclusive, we must add 1 to this solution, which give us $\boxed{\textbf{(E) }58}$.

Video Solutions




- MathEx




https://www.youtube.com/watch?v=r0e_6dXViRI ~David

Video Solution by OmegaLearn


See Also

2018 AMC 8 (ProblemsAnswer KeyResources)
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Problem 24
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