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Bunuel
How many three-digit positive integers are multiples of 7 but not multiples of 6 or 15 ?

A. 98
B. 102
C. 106
D. 110
E. 114


 


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We need 3-digit positive integers that are multiples of 7 but not multiples of 6 or 15
Multiples of 7 are 7, 14, 21, 28, 35, etc.
-Smallest 3-digit number divisible by 7 = 105
-Largest 3-digit number divisible by 7 = 994
-994 = 105 + 7(n-1) [where 7 is the common difference and n is number of multiple of 7.]
-n = 128
Multiples of 6 are 6, 12, 18, 24, etc.
Multiples of 15 are 15, 30, 45, etc.

There are 26 such numbers
So we can rule out multiples of 6 and 15. The remaining multiples of 7 between 100 and 999 are 128-26=102
B
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Asked: How many three-digit positive integers are multiples of 7 but not multiples of 6 or 15 ?

Number of 3-digit positive integers that are multiple of 7 = {105,....994} = (994-105)/7 + 1 = 889/7 + 1 = 127 + 1 = 128
Number of 3-digit positive integers that are multiple of (7 & 6 = 42) = {126,...,966} = (966-126)/42 + 1 = 840/42 + 1 = 21
Number of 3-digit positive integers that are multiple of (7 & 15 = 105) = {105,...., 945} = (945-105)/105 + 1 = 840/105 + 1 = 9
Number of 3-digit positive integers that are multiple of (7 & 30 = 210) = {210,...., 840} = (840-210)/210 + 1 = 630/210 + 1 = 4

Number of three-digit positive integers are multiples of 7 but not multiples of 6 or 15 = 128 - (21+9-4) = 128 - 26 = 102

IMO B
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Bunuel
How many three-digit positive integers are multiples of 7 but not multiples of 6 or 15 ?

A. 98
B. 102
C. 106
D. 110
E. 114


 


This question was provided by GMAT Club
for the Around the World in 80 Questions

Win over $20,000 in prizes: Courses, Tests & more

 


smallest 3-D number divisible by 7= 105,
total number of 3 digit nos. divisible by 7 are,
using AP formula we get, 994=105+(n-1)*7
n= 128.

Now, find out total numbers divisible by 6*7= 42 and 15*7= 105 separately.
we get,
for 42, 966=126+(n-1)*42
n=21.

for 105, 945=105+(n-1)*105
n=9.

Now take LCM of 6,15, we get= 30, now multiply 30*7=210,
Now calculate 3 digit numbers divisible by 210- no need for formula here, we can calculate manually- 210, 420, 630, 840.

Only divisible by 42 = 21
Only divisible by 105 =9
divisible by both 42 and 105 =4
Total= 21+(9-4)= 26,

Deduct 26 from 128 (3 digit numbers divisible by 7)
Hence, 128-26= 102.

Answer: 102 (B)
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7* 15 =105
7*142=994
so total number of 7s multiples are (142-15)+1=128

Now we have to remove multiple of 6s, 15s and common of 6and 15.

7*18 is a multiple of 6.
6*3=18
6*23=138

total multiples of 6 are (23-3)+1=21 ......... (A)
7*15 is a multiple of 6.
siilarly 7*30, 7*45, 7*60, 7*75, 7*90,7*105,7*120 and 7*135 are multiple of 6.
total multiple of 15 are 9. ... (B)

now 7*30, 7*60,7*90,7*120 are common. total common are 4. we ned to add these.

128-A-B+4=128-21-9+4=102.
Option B
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Bunuel
How many three-digit positive integers are multiples of 7 but not multiples of 6 or 15 ?

A. 98
B. 102
C. 106
D. 110
E. 114


 


This question was provided by GMAT Club
for the Around the World in 80 Questions

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# multiples of 7 - # multiples of (7 U 6) - # multiples (7 U 15) + # multiples (7 U 6 U 15), (multiples of 6 and 7 can concide).
# of items = [(Last - First) / increment] + 1
# multiples of 7, [(994 - 105) / 7] + 1 = 128
# multiples (7 U 6), [(966 - 126) / 42] + 1 = 21
# multiples (7 U 15), [(945 - 105) / 105] + 1 = 9
# multiples (7 U 6 U 15), [(840 - 210) / 210] + 1 = 4
# multiples of 7 - # multiples of (7 U 6) - # multiples (7 U 15) + # multiples (7 U 6 U 15) = 128 - 21 - 9 + 4 = 102
Answer 102

B
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Bunuel
How many three-digit positive integers are multiples of 7 but not multiples of 6 or 15 ?
A. 98
B. 102
C. 106
D. 110
E. 114
 


This question was provided by GMAT Club
for the Around the World in 80 Questions

Win over $20,000 in prizes: Courses, Tests & more

 


The first 3-digit multiple of 7 is 105 = 7 * 15
The last 3-digit multiple of 7 is 994 = 7 * 142 (dividing 999 by 7, we get the quotient and remainder; remove the remainder from 999; alternatively you know that 1001 is a multiple of 7)
So, the required multiples are 15, 16, 17 ... 142 => 142 - 15 + 1 = 128 numbers
We do not want the multiples of 6 or 15
Number of multiples of 6 are: 7*18 ... 7*138 => (138 - 18)/6 + 1 = 21 numbers
Number of multiples of 15 are: 7*15 ... 7*135 => (135 - 15)/15 + 1 = 9 numbers
Number of multiples of 6 and 15, i.e. 30 are: 7*30, 7*60, 7*90 and 7*120 => 4 numbers
Thus, we need to remove 21 + 9 - 4 = 26 numbers from the 128 numbers

Required number = 128 - 26 = 102
Answer B
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