Mind Your 5's And 7's

Ammar is thinking of a set of 10 distinct positive integers. He see that exactly 5 of them are divisible by 5, and exactly 7 of them are divisible by 7. If M is the greatest of these 10 numbers, what is the minimum value of M?

77 63 75 70

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10 solutions

Discussions for this problem are now closed

Biprasish Ray
May 12, 2014

It is given that exactly 5 nos are divisible by 5 and exactly 7nos are divisible by 7. So, there are two nos which are divisible by both 5 and 7. The least possible of such nos are 35 and 70. Hence the value of M is 70

Sorry.. i not understand.. can you explain to me? Thanks.. Why the greatest number must be divided by both 5 and 7 but not either one only?

Khoo Siang - 7 years, 1 month ago

There are 5 numbers divisible by 5 and 7 numbers divisible by 7. 5+7=12. But, there are only 10 numbers in the set, therefore there must be 2 numbers divisible by both, and the smallest multiples are 35 and 70, hence the answer is 70.

Jim Zhou - 7 years, 1 month ago

Thank You :D

Agnishom Chattopadhyay - 7 years, 1 month ago

Why 70 is tge greatest number is not clear.

Samir Shaheen - 7 years, 1 month ago

5 no should be 5x and 7 no should be 7x means minimum 2 no will be 35x hence The least possible of such nos are 35 and 70. Hence the value of M is 70

Jayraj Kanabar - 7 years, 1 month ago
Connor Kenway
May 14, 2014

Anyone who is thinking that there are infinitely many solutions to this think again the problem is asking for the MINIMUM possible value of the greatest number on the list and as others have explained we have to have 2 numbers divisible by both 5 and 7 that is divisible by 35 so we have to have 35 and 70 on the list.We could have had 70 and 105 also but the question is asking the minimum value of the greatest number.The rest of the numbers that is the 3 numbers that have to divisible by only 5 and the 5 numbers that have to be divisible by only 7 can be choosen from numbers smaller than 70. To understand this better you need to think in terms of sets i.e N(A union B)=N(A)+N(B)-N(A intersection B)

Shirley Miao
May 19, 2014

There are 5 numbers that are divisible by 5, and 7 numbers divisible by 7. If these were all distinct (i.e. no number is divisible by both), there could not be only 10 numbers, since 5 + 7 = 12 numbers. Thus, at least two numbers are divisible by both 5 and 7.

The first of these is 35. The second is 70. 70 is the least number we know has to be included in the 10 numbers, so we can set M = 70.

Being Parthu
May 14, 2014

it is specified that there are 5num divisible by 5 and 7 numbers divisible by 7 so num may be 5,10,15,7 ,14,21,28,42,35,70

Muhammad Ahmad
May 19, 2014

The required set of Ammar may be: {5,7,10,14,15,21,28,35,42,70} Here the elements {5,10,15,35,70} are exactly 5 integers that are divisible by 5 and the elements {7,14,21,28,35,42,70} , two elements must be common multiples of 5 and 7 so here {35,70} are common multiples.

Srikanth Movva
May 18, 2014

very simple. total numbers=10 5=5 and *7=7. so 2 numbers r common. common numbers =35(5 7) and 70(5 7 2) [least 2 common multiples] in remaining 8 numbers,4 r multiples of 5 and 4 r of 7. the question is asked such that M is min.so the answer is 70 :)

Zack Yeung
May 15, 2014

Find LCM = 35.....but there is no 35....so x 2...70...which can be divided by 5 n 7

Emran Tahir
May 15, 2014

10 distinct positive integers!!! at least 2 numbers divisible with both 5 and 7..... these two would be ....0 , 70

Manish Patwal
May 13, 2014

Total 10 integers are there hence two integers should be multiples of both 5 and 7.. That is 35 and 70.. Greatest of them is 70. Other 8 integers could be 5,10,15,7 ,14,21,28,42.

T Satish Kumar
May 13, 2014

it is given 5 nos are divisible by 5 and 7 nos are divisible by 7 so there is minimum 2 nos divisible by both 5 and 7 . so least value for that is 35 and 70. thus minimum value of largest no. is 70.

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