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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
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If r is an integer, and 700 < 10!/(10r)!<1,000, then r=
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21 Nov 2017, 17:23
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[GMAT math practice question] If r is an integer, and \(700 < \frac{10!}{(10r)!}<1,000\), then r= A. 1 B. 2 C. 3 D. 4 E. 5
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Re: If r is an integer, and 700 < 10!/(10r)!<1,000, then r=
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21 Nov 2017, 18:37
MathRevolution wrote: [GMAT math practice question]
If r is an integer, and \(700 < \frac{10!}{(10r)!}<1,000\), then r= A. 1 B. 2 C. 3 D. 4 E. 5 Hi.... \(\frac{10!}{(10r)!}=10*9*8..*(10(r1)\) Now it is given this value is more than 700 and less than 1000 10*9*8=720 So 10(r1)=8.....r1=108=2....r=2+1=3 C
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Re: If r is an integer, and 700 < 10!/(10r)!<1,000, then r=
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22 Nov 2017, 04:27
MathRevolution wrote: [GMAT math practice question]
If r is an integer, and \(700 < \frac{10!}{(10r)!}<1,000\), then r= A. 1 B. 2 C. 3 D. 4 E. 5 An easy way to solve this question is to plug in the answers given, calculate the value of \(\frac{10!}{(10r)!}\) and see whether it satisfies or violates the inequality in the problem prompt. A/\(r = 1\) This gives us: \(\frac{10!}{(10r)!} = \frac{10!}{9!} = 10\). This violates the inequality given in the problem prompt. B/ \(r = 2\) This gives us: \(\frac{10!}{(10r)!} = \frac{10!}{8!} = 10*9 = 90\). This also violates the inequality given in the problem prompt. C/ \(r = 3\) This gives us: \(\frac{10!}{(10r)!} = \frac{10!}{7!} = 10*9*8 = 720\). This satisfies the inequality given in the problem prompt, therefore, this is our answer (we can afford to stop here since this is not a DS question nor is the unknown a polynomial/noninteger with multiple potential values).Thus, the answer is C.



Math Revolution GMAT Instructor
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GPA: 3.82

If r is an integer, and 700 < 10!/(10r)!<1,000, then r=
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22 Nov 2017, 18:54
=> Note that \(\frac{10!}{(10r)!} = 10 * (101) * … * (10r+1)\). Test different values of \(r\): \(r=1 : 10\) \(r=2 : 10 * 9 = 90\) \(r=3 : 10 * 9 * 8 = 720\) \(r=4 : 10 * 9 * 8 * 7 = 5040\) Therefore,\(r = 3\). The answer is C. Answer: C
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Re: If r is an integer, and 700 < 10!/(10r)!<1,000, then r=
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06 Feb 2018, 10:55
chetan2u wrote: MathRevolution wrote: [GMAT math practice question]
If r is an integer, and \(700 < \frac{10!}{(10r)!}<1,000\), then r= A. 1 B. 2 C. 3 D. 4 E. 5 Hi.... \(\frac{10!}{(10r)!}=10*9*8..*(10(r1))\) Now it is given this value is more than 700 and less than 1000 10*9*8=720 So 10(r1)=8.....r1=108=2....r=2+1=3 C Hi Chetan, Can you please explain how did you derive \(\frac{10!}{(10r)!}=10*9*8..*(10(r1)\)




Re: If r is an integer, and 700 < 10!/(10r)!<1,000, then r= &nbs
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06 Feb 2018, 10:55






