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How many four digit numbers are there such that the last two digit gro

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How many four digit numbers are there such that the last two digit gro  [#permalink]

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New post 12 Nov 2018, 22:00
6
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A
B
C
D
E

Difficulty:

  55% (hard)

Question Stats:

53% (02:05) correct 47% (01:32) wrong based on 57 sessions

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How many four digit numbers are there such that the last two digit grouping is three times the first two digit grouping?

A. 12
B. 18
C. 24
D. 30
E. 36
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Posts: 7106
Re: How many four digit numbers are there such that the last two digit gro  [#permalink]

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New post 12 Nov 2018, 22:20
gracie wrote:
How many four digit numbers are there such that the last two digit grouping is three times the first two digit grouping?

A. 12
B. 18
C. 24
D. 30
E. 36


let number be ABCD
Let us take restrictions on the first digit from left, A - it cannot be greater than 3 as 4_*3 will result in a 3-digit number and it cannot be 0
1) let A be 1, B can be anything from 0 to 9 as even the largest 19*3=57 < 100....so 10 ways
2) let A be 2, B can be anything from 0 to 9 as even the largest 29*3=87 < 100....so 10 ways
3) let A be 3, B can be anything from 0 to 3 as 34*3=102, a 3-digit number and cannot be equal to CD....so 4 ways

total = 10+10+4=24

C
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Re: How many four digit numbers are there such that the last two digit gro  [#permalink]

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New post 12 Nov 2018, 22:51
chetan2u wrote:
gracie wrote:
How many four digit numbers are there such that the last two digit grouping is three times the first two digit grouping?

A. 12
B. 18
C. 24
D. 30
E. 36


let number be ABCD
Let us take restrictions on the first digit from left, A - it cannot be greater than 3 as 4_*3 will result in a 3-digit number and it cannot be 0
1) let A be 1, B can be anything from 0 to 9 as even the largest 19*3=57 < 100....so 10 ways
2) let A be 2, B can be anything from 0 to 9 as even the largest 29*3=87 < 100....so 10 ways
3) let A be 3, B can be anything from 0 to 3 as 34*3=102, a 3-digit number and cannot be equal to CD....so 4 ways

total = 10+10+4=24

C


According to me correct choice would be D. 30
In addition to the 24 nos which you explained there are 6 more..

4. Let A be 4-9 and B =0; 4012, 5015, 6018, 7021, 8024, 9027.

Total 10+10+4+6 = 30
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Re: How many four digit numbers are there such that the last two digit gro  [#permalink]

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New post 23 Nov 2018, 02:04
rintumitra

I think it's a meaning problem.

AB=40 does not equal "04" but equals "40". Therefore if AB=40, CD=120, which is not possible since we are talking about 4 digit integers.
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Re: How many four digit numbers are there such that the last two digit gro  [#permalink]

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New post 23 Nov 2018, 18:46
1
Choice C is correct.

Range of first 2 digits can be from 10 to 33, inclusive only as 00-09 is not possible since it's a four digit number and anything above 33 will yield last 2 digits as more than 100. So range of numbers = 33-10+1 = 24.
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Re: How many four digit numbers are there such that the last two digit gro &nbs [#permalink] 23 Nov 2018, 18:46
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