Yay for 2014! 5

Find the sum of the digits of the smallest positive multiple of 14 that leaves a remainder of 14 when divided by 20 and a remainder of 4 when divided by 201.


This problem is part of the set Yay for 2014! .


The answer is 13.

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3 solutions

Alex Wang
Nov 6, 2014

We can write: 14 k 14 ( m o d 20 ) 14k \equiv 14 \pmod{20} where k is an integer. Which simplifies to k 1 ( m o d 10 ) . k \equiv 1 \pmod{10}.

Now, from the second condition, we write: 14 k 4 ( m o d 201 ) 14k \equiv 4 \pmod{201} k 29 ( m o d 201 ) . k \equiv 29 \pmod{201}.

The smallest k that satisfies these conditions is k=431. Then, our number is 431*14=6034.

Therefore, the sum of the digits is 6 + 0 + 3 + 4 = 13 6+0+3+4=\boxed{13} .

cant understand how u came to the second step.....that (mod 10 ) thing

prajwal kavad - 6 years, 4 months ago
Adarsh Kumar
Nov 3, 2014

F r o m t h e g i v e n d a t a w e d e r i v e t h e f o l l o w i n g e q u a t i o n s : 14 x 14 = 20 a 14 x 4 = 201 b . N o w , s u b t r a c t t h e a b o v e t w o e q u a t i o n s t o g e t : 10 = 20 a 201 b 10 ( 1 + 2 a ) = 201 b . N o w , b e c a u s e g c d ( 201 , 10 ) = 1 , t h e v a l u e s o f ( 1 + 2 a ) a r e o f t h e f o r m 201 y . T h u s , ( 1 + 2 a ) = 201 y . S u b s t i t u t i n g t h i s v a l u e i n t h e f i r s t e q u a t i o n g i v e s : 14 x 14 = 2010 y 10 14 x = 2010 y + 4. B u t , 2010 = 14 p + 8 ( p = 143 ) 14 x = 14 a y + 8 y + 4. 8 y + 4 m u s t b e d i v i s i b l e b y 14. T h e s m a l l e s t v a l u e o f y t h a t s a t i s f i e s t h i s i s 3. 14 x = 6034. From\ the\ given\ data\ we\ derive\ the\ following\ equations:\\ 14x-14=20a\\ 14x-4=201b.\\Now,subtract\ the\ above\ two\ equations\ to\ get:\\ 10=20a-201b\\ \Rightarrow10(1+2a)=201b.\\Now,because\ gcd(201,10)=1,\\the\ values\ of\ (1+2a)\ are\ of\ the\ form\ 201y.Thus,(1+2a)=201y.\\Substituting\ this\ value\ in\ the\ first\ equation\ gives:\\ 14x-14=2010y-10\\ \Rightarrow 14x=2010y+4.\\ But,2010=14p+8(p=143)\\ \Rightarrow 14x=14ay+8y+4.\\ \Rightarrow 8y+4\ must\ be\ divisible\ by\ 14.The\ smallest\ value\ of\ y\ that\ satisfies\ this\ \\is\ 3.\\ \Rightarrow 14x=6034.

Ar Agarwal
Nov 4, 2014

Here's a simple Python 3.0 script to achieve the same:

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n = 10
digit_sum = 0
while(True):
    #this condition is because on dividing by 20 it leaves remainder
    #as 14, so its last digit should be 4. Also on dividing by 201, it
    #leaves remainder as 4 so the previous multiple of 201 must be obtained by
    #multiplying 201 with a multiple of 10. However, we increase by 20 every
    #because on increase of 10 we would obtain a number which on dividing
    #by 20 leaves 4 as remainder. So, we consider only odd multiples of 10.
    #Now, two conditions are covered adding 4 and checking mod against 14 
    #checks for 3rd condition.
    if((n*201+4)%14==0):
        break
    n = n + 20
num = n*201+4
while(num>0):
    digit_sum = digit_sum + num%10
    num = num//10
print(digit_sum)

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