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Four different prime numbers, each less than 20, are multiplied togeth

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Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 23 Feb 2017, 19:27
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A
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Question Stats:

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Four different prime numbers, each less than 20, are multiplied together. What is the greatest possible result?
○A○ 21,879
○B○ 28,728
○C○ 40,755
○D○ 46,189
○E○ 49,742
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Re: Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 23 Feb 2017, 21:51
vikasp99 wrote:
Four different prime numbers, each less than 20, are multiplied together. What is the greatest possible result?
○A○ 21,879
○B○ 28,728
○C○ 40,755
○D○ 46,189
○E○ 49,742


Prime numbers less than 20 are {2,3,5,7,11,13,17,19}

Biggest required product = 11*13*17*19=46189

Answer Option D
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Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 23 Feb 2017, 22:05
vikasp99 wrote:
Four different prime numbers, each less than 20, are multiplied together. What is the greatest possible result?
○A○ 21,879
○B○ 28,728
○C○ 40,755
○D○ 46,189
○E○ 49,742


To get the greatest possible result, we must consider prime numbers 19, 17, 13 and 11.
The product of the units digit of each of the above numbers => 9*7*3*1 will result in a number with 9 at its unit digit (Only A and D are possible options)
Use approximation to get 19*17*13*11 => 20*20*10*10 = 40000 (D is close enough)

Hence option D is correct
Hit Kudos if you liked it 8-)
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Re: Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 26 Feb 2017, 11:59
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Hi All,

This question is built around several Number Properties - and the answer choices are 'spread out' enough that you don't really have to do a lot of 'heavy math' to get to the correct answer.

To start, we have to identify the 4 largest PRIMES that are less than 20. They are: 19, 17, 13 and 11.

Next, notice how they are all ODD integers. When we take the product of 4 odd numbers, we end up with an ODD number. Eliminate Answers B and E. Since none of the 4 primes ends in a '5', the product will NOT end in a '5' - so we can eliminate Answer C.

There is such a big difference in the two remaining answers (21,879 and 46,189) that we can estimate the product. Let's 'round up' the 19 and 17 to 20 and 20. Let's 'round down' the 13 and 11 to 10 and 10. This gives us....

(20(20)(10)(10) = (400)(100) = 40,000

Final Answer:

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Re: Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 03 Apr 2017, 04:54
Great way to solve Rich!
I solved it by a rather conventional method:
To maximize the product, take the bigger nos. (prime nos.)
11,13,17,19
Their product is 46189.
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Re: Four different prime numbers, each less than 20, are multiplied togeth  [#permalink]

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New post 09 Jan 2019, 10:37
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: Four different prime numbers, each less than 20, are multiplied togeth &nbs [#permalink] 09 Jan 2019, 10:37
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