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.
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cant understand how u came to the second step.....that (mod 10 ) thing
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 : 1 4 x − 1 4 = 2 0 a 1 4 x − 4 = 2 0 1 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 : 1 0 = 2 0 a − 2 0 1 b ⇒ 1 0 ( 1 + 2 a ) = 2 0 1 b . N o w , b e c a u s e g c d ( 2 0 1 , 1 0 ) = 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 2 0 1 y . T h u s , ( 1 + 2 a ) = 2 0 1 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 : 1 4 x − 1 4 = 2 0 1 0 y − 1 0 ⇒ 1 4 x = 2 0 1 0 y + 4 . B u t , 2 0 1 0 = 1 4 p + 8 ( p = 1 4 3 ) ⇒ 1 4 x = 1 4 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 1 4 . 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 . ⇒ 1 4 x = 6 0 3 4 .
Here's a simple Python 3.0 script to achieve the same:
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We can write: 1 4 k ≡ 1 4 ( m o d 2 0 ) where k is an integer. Which simplifies to k ≡ 1 ( m o d 1 0 ) .
Now, from the second condition, we write: 1 4 k ≡ 4 ( m o d 2 0 1 ) k ≡ 2 9 ( m o d 2 0 1 ) .
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 = 1 3 .