Does it work for some other larger number too?

Algebra Level 1

Fill in the blank:

1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 1,2,3,4,5,6,7,8,9,10 The above are the first 10 positive integers. The product of the numbers before the number _____ \text{\_\_\_\_\_} is the same as the product of the numbers after that number.

7 5 6 8

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4 solutions

Gregory Lewis
Sep 12, 2017

Since there is only one factor of 7, we immediately know that must be the answer (if one exists), because 7 could not belong to either group. Verification is trivial.

Verification is fairly trivial, so I wonder why you didn't do it? 6! = 720. 8 x 9 x 10 = 720. Regards, David

David Fairer - 3 years, 8 months ago
Munem Shahriar
Sep 12, 2017

The condition is, the product of the numbers before the number ___ \text{\_\_\_} is the same as the product of the numbers after that number.

Only 7 \color{#3D99F6} \boxed{7} satisfies the condition.

1 × 2 × 3 × 4 × 5 × 6 × 7 = 5040 \large \color{#D61F06} \Rightarrow \color{#3D99F6}1 \times 2 \times 3 \times 4 \times 5 \times 6 \times 7 = \color{#20A900}5040

7 × 8 × 9 × 10 = 5040 \large \color{#D61F06} \Rightarrow \color{#3D99F6} 7 \times 8 \times 9 \times 10 = \color{#20A900}5040

Hence the product of the numbers before the number 7 \color{#3D99F6} \boxed{7} is the same as the product of the numbers after that number.

Kenny O.
Sep 13, 2017

The product of the numbers from 1 to 10 is 10!. We let the number fit to be in the blank be n. As both the products of the number before n and after n are equal (and an integer), 10!/n is a square number.
The only number satisfying this is 7
We then check is 7 is correct. 6!=720= 8 9 10 8*9*10

Naren Bhandari
Sep 25, 2017

Let the product be p = k = 1 10 k = 1 × 2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 10 \text{p} = \displaystyle\prod_{k=1}^{10}k = 1\times 2\times 3\times 4\times 5\times 6\times 7\times 8\times 9\times 10 here 4 , 6 , 8 , 9 , 10 4,6,8,9,10 are composite numbers that has the prime factor of 2 , 3 , 5 2,3,5 and there doesn't exist any number that can be expressed as the prime factor of 7 7 . So, 7 7 is the intermediate number in which the product of number before it and after it is equal. p = k = 1 10 = 1 × 2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 10 = 2 8 × 3 4 × 5 2 × 7 1 \text{p} =\displaystyle\prod_{k=1}^{10} =1\times 2\times 3\times 4\times 5\times 6\times 7\times 8\times 9\times 10 =2^8\times 3^4\times 5^2\times 7^1 . The power to the prime are in geometric progression for k = 1 10 k \displaystyle\prod_{k=1}^{10}k however,for k > 10 k>10 doesn't form G.P which fails for other such numbers.

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