Numbers

How many positive perfect squares less than 1 0 6 10^6 are multiples of 24?


Source: 2007 AIME I.


The answer is 83.

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3 solutions

Department 8
Nov 19, 2015

The prime factorization of 24 24 is 2 3 × 3 2^3\times 3 . Thus, each square must have at least 3 3 factors of 2 2 and 1 1 factor of 3 3 and its square root must have 2 2 factors of 2 2 and 1 1 factor of 3 3 . This means that each square is in the form ( 12 c ) 2 (12c)^2 , where c c is a positive integer less than 1 0 6 \sqrt{10^6} . There are 1000 12 = 83 \left\lfloor \frac{1000}{12}\right\rfloor = \boxed{83} solutions.

nice observation...

Dev Sharma - 5 years, 6 months ago

Nicely done !! I did the same too !!

Akshat Sharda - 5 years, 6 months ago

Same way bro

Yellow Tomato - 5 years, 6 months ago
Zee Ell
Dec 23, 2015

24 = 2 2 × 6 24=2^2×6

1 0 6 = 1 0 3 = 1000 \sqrt {10^6}=10^3=1000

1000÷2=500

500 6 = 83 \lfloor \frac{500}{6}\rfloor= \boxed {83}

Lee Isaac
Nov 19, 2015

24=2^3 \times 3

For a perfect square to be a multiple of 24, it has to be a multiple of 2^4 \times 3^4=144.

10^6\144 \approx 6944.44.

Now we need to find the number of perfect squares below 6944.44.

\sqrt{6944.44} \approx 83.33

\lfloor 83.33 \rfloor = \boxed{83}

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Lu Chee Ket - 5 years, 6 months ago

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(sqrt{100}

It's not working, could you explain more? Thanks.

Lee Isaac - 5 years, 6 months ago

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100 \sqrt{100} by typing \ ( \sqrt{100} \ ) with no gap of space for critical places.

Lu Chee Ket - 5 years, 6 months ago

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