Fill in the blank:
1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 1 0 The above are the first 10 positive integers. The product of the numbers before the number _____ is the same as the product of the numbers after that number.
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Verification is fairly trivial, so I wonder why you didn't do it? 6! = 720. 8 x 9 x 10 = 720. Regards, David
The condition is, the product of the numbers before the number ___ is the same as the product of the numbers after that number.
Only 7 satisfies the condition.
⇒ 1 × 2 × 3 × 4 × 5 × 6 × 7 = 5 0 4 0
⇒ 7 × 8 × 9 × 1 0 = 5 0 4 0
Hence the product of the numbers before the number 7 is the same as the product of the numbers after that number.
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
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9
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Let the product be p = k = 1 ∏ 1 0 k = 1 × 2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 1 0 here 4 , 6 , 8 , 9 , 1 0 are composite numbers that has the prime factor of 2 , 3 , 5 and there doesn't exist any number that can be expressed as the prime factor of 7 . So, 7 is the intermediate number in which the product of number before it and after it is equal. p = k = 1 ∏ 1 0 = 1 × 2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 1 0 = 2 8 × 3 4 × 5 2 × 7 1 . The power to the prime are in geometric progression for k = 1 ∏ 1 0 k however,for k > 1 0 doesn't form G.P which fails for other such numbers.
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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.