Algebra or Simple Counting?

How many 2- digit numbers are divisible by 7?


The answer is 13.

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6 solutions

Nihar Mahajan
Sep 8, 2015

The total numbers divisible by 7 7 below 100 100 would be given by 100 7 = 14 \left \lfloor \dfrac{100}{7} \right \rfloor = 14 . But one of them is 7 7 which is one-digit number. Thus the number of required two digit numbers = 14 1 = 13 =14-1 = \large\boxed{13}

Nice approach, please do review mine, as I have done it a different way :)

Swapnil Das - 5 years, 9 months ago

I like this math explanation, but the question doesn't say how many are evenly divisible by 7. I can divide all the 2 digit numbers by 7 and tell you the remainder if you like.

Nelson Brentlinger - 5 years, 9 months ago

Nice explanation but I am confused or may be other people, I did not understand why you compared 100 since in question it is 7. ??

I may seem odd ............ :P

Syed Baqir - 5 years, 9 months ago

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100 is the first 3 digit number, therefore all the numbers needed for the solution must be between 14 ( the first 2 digit multiple of 7) and 100 (the first 3 digit number)

Sarah Webb - 5 years, 8 months ago

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Thanks for the reply.. I got it now

Syed Baqir - 5 years, 7 months ago

Good solution

Rama Devi - 5 years, 9 months ago
Swapnil Das
Sep 8, 2015

The very first 2 2 digit multiple of 7 7 is 14 14 and the 98 98 is the last. The only thing we have to do is to find the number of terms of this arithmetic progression.

a = 14 a = 14 , d = 7 d= 7

a n = a + ( n 1 ) d { a }_{ n } = a + (n-1)d

98 = 14 + ( n 1 ) 7 \Rightarrow 98 = 14 + (n-1)7

n = 13 \Rightarrow n= 13

Yes , this is nice too :)

Nihar Mahajan - 5 years, 9 months ago

Can you tell us why you are finding multiple of 7 .

I mean 7/2 = decimal , 7/4 = decimal

And so on ........

Syed Baqir - 5 years, 9 months ago

This one seems nice. But here also we have to calculate the last number which is divisible by 7.

Rajalaxmi Mishra - 5 years, 9 months ago

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Ya, that's the only effort we make. Thanks!

Swapnil Das - 5 years, 9 months ago

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You are most welcome Swapnil sir..

Rajalaxmi Mishra - 5 years, 9 months ago

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@Rajalaxmi Mishra No sir please.. I don't deserve that title.

Swapnil Das - 5 years, 9 months ago

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@Swapnil Das Ohk..I will not call you sir..But I thought "sir" is a perfect word to address a man..Isn't it?

Rajalaxmi Mishra - 5 years, 9 months ago

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@Rajalaxmi Mishra I'm younger than you, so my name is enough, I guess.

Swapnil Das - 5 years, 9 months ago

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@Swapnil Das Ohk Swapnil..

Rajalaxmi Mishra - 5 years, 9 months ago

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@Rajalaxmi Mishra Haha, that's better Didi😛

Swapnil Das - 5 years, 9 months ago

Like this one

Ken Collins - 5 years, 9 months ago
Puneet Agrawal
Sep 18, 2015

Divide by 7 with highest divisible no. of 7 and then subtracted by 1

Nirupam Choudhury
Sep 11, 2015

In the table of 7 the highest no by which the answer is in two digit excluding 1 because 7 is one digit So the answer is 13

Shubham Beri
Sep 9, 2015

As we know, 98 is the last two digits no divisible by 7 98/7 = 14 but in these 14 numbers we have also counted 7 which is of one digit i.e. we will substract this case from 14, 14 - 1 = 13

Chris Nixon
Sep 9, 2015

Why aren't we including negatives? This would count all two-digit numbers between -99 and 99, meaning the solution should be 26.

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