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It's a hard question... for my thinking! I've tried many solutions but haven't figured it out. (Let $M$ be the unknown number)

I've tried:

$M^3-8=-5$

Then

$M^3-8+8=5+8$

Then I think the answer is $M^3=13$

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    $\begingroup$ Triple a number is not the same as the number squared. Instead your equation should be $3M-8=-5$. $\endgroup$ – René B. Christensen May 22 '14 at 10:26
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    $\begingroup$ @ René B. Christensen: I think "cubed" is the word you are looking for. $\endgroup$ – abstract May 22 '14 at 10:30
  • $\begingroup$ @abstract Yes, of course. I might have written that comment a bit too fast. $\endgroup$ – René B. Christensen May 22 '14 at 10:37
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Hint: A number is tripled if it is multiplied by $3$, not put to the third power. For example, tripling the number $5$ gives $15$.

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