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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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=>

2, 3, 5, 7, 11, 13, 17, 19, 23 and 29 are the first 10 prime numbers.
Of these, only 2 is an even integer. Their product is an even number, but it is not divisible by 4.
Thus, the product has remainder 2 when it is divided by 4.

Therefore, C is the answer.
Answer: C
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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MathRevolution wrote:
[GMAT math practice question]

What is the remainder when the product of the first 10 prime numbers is divided by 4?

A. 0
B. 1
C. 2
D. 3
E. not defined


because 2 and 5 are factors, product units digit is 0
because 2 is the only even factor, product is not divisible by 4
thus, only option for remainder is 2
C
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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gmat1393 wrote:
Hi chetan2u Can you please point where am I going wrong in this approach?
gmat1393 wrote:
Hi MathRevolution

Doesnt the question stem reduce to

(2*3*5*7*11*13*17*19*23*29)/ 4

If we cancel out 2 from Numerator and denominator.
We have 2 in denominator.
So remainder should be 1.

What is wrong in this approach.Can you please help?

Posted from my mobile device




This approach is wrong for the same reason the remainder of 10/6 (or (2*5)/(2*3)) is not equal to the remainder of 5/3.
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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MathRevolution wrote:
[GMAT math practice question]

What is the remainder when the product of the first 10 prime numbers is divided by 4?

A. 0
B. 1
C. 2
D. 3
E. not defined


Product of first 10 prime numbers = 2*3*5*...

When divided by 4, we need remainder for this calculation: 2*3*5*... / 4

Reduce the fraction to get 3*5*... / 2

Since all other prime numbers will be odd, the product will be odd too. So upon division by 2, the remainder will be 1.

To get the remainder upon division by 4, we need to multiply the 2 back to the remainder.

Actual remainder = 1*2 = 2

Answer (C)
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
Hi MathRevolution

Doesnt the question stem reduce to

(2*3*5*7*11*13*17*19*23*29)/ 4

If we cancel out 2 from Numerator and denominator.
We have 2 in denominator.
So remainder should be 1.

What is wrong in this approach.Can you please help?

Posted from my mobile device
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
Hi chetan2u Can you please point where am I going wrong in this approach?
gmat1393 wrote:
Hi MathRevolution

Doesnt the question stem reduce to

(2*3*5*7*11*13*17*19*23*29)/ 4

If we cancel out 2 from Numerator and denominator.
We have 2 in denominator.
So remainder should be 1.

What is wrong in this approach.Can you please help?

Posted from my mobile device
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
First 10 prime numbers are 2*3*5*7*11*13*17*19*23*29

We can see them as

2*3*x, where x= 5*7*11*13*17*19*23*29

Then it can be expressed as 6x.
The remainder of 6/4 is 2.
6x = (4x +2x)
4x/4 has a remainder of 0 and 2x/4 will always have a remainder of 2 since
a) x is not a multiple of 2 because it is a product of only prime numbers
b)the remainder cannot be three because we're dealing with integers only (x is not 1.5)
c) even if one or more of the remainders of the factors of x (5, 7, etc) was three (3*3*3...) or one, this number would be mutiplied by 2 and the remainder, again, has to be two.
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
Essentially we need to find remainder when 3*5*.... *29 is divided by 2. The remainder is always less than divisor itself therefore answer can be only 0 or 1. Now as there is no factor of 2 left, therefore remainder will be 1


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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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gmat1393 wrote:
Hi chetan2u Can you please point where am I going wrong in this approach?
gmat1393 wrote:
Hi MathRevolution

Doesnt the question stem reduce to

(2*3*5*7*11*13*17*19*23*29)/ 4

If we cancel out 2 from Numerator and denominator.
We have 2 in denominator.
So remainder should be 1.

What is wrong in this approach.Can you please help?

Posted from my mobile device



2*3*5*7*11*13*17*19*23*29 = 6469693230 = 4*1617423307 + 2

When 2*3*5*7*11*13*17*19*23*29 = 6469693230 is divided by 4, its remainder is 2.

We can define a remainder in the following way.
If we have A = B*Q + R where 0 ≤ R < |B|, R is the remainder when A is divided by B.
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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I solved this question in about 50 seconds.
This is how I do it.

We know the first 10 prime numbers include 2 and 9 odd numbers.
So the product of those numbers must be even but not divisible by 4.
Hence, the remainder when divided by 4 is always 2.

Let's try some easy numbers.
Product of first prime number : 2
Remainder when divided by 4 : 2
Product of first 2 prime numbers : 2 x 3 = 6
Remainder when divided by 4 : 2
Product of first 3 prime numbers : 2 x 3 x 5 = 30
Remainder when divided by 4 : 2
Product of first 4 prime numbers : 2 x 3 x 5 x 7 = 210
Remainder when divided by 4 : 2

Remainder is always 2.
Hence the answer is C.
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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
First prime number is 2 and so divided by 4 reduces the question to:

Odd number/2

Only remainder on this basis is 1, but remainders are referenced to an unreduced denominator, so 1 is to 2 as

2 is to 4. 2.

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Re: What is the remainder when the product of the first 10 prime numbers i [#permalink]
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