abcab is a multiple of 7
abc is the multiple of 9
cba is the multiple of 4
what is the minimum of a x b x c ? (a,b,c >0)
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a b c a b = a × 1 0 4 + b × 1 0 3 + c × 1 0 2 + a × 1 0 + b
Which gives modulo 7, and using first property:
4 a + 6 b + 2 c + 3 a + b = 2 c = 0 ( m o d 7 )
Since 0 < c < 1 0 we deduce c = 7
From second property we deduce a + b + 7 = 0 ( m o d 9 )
Since 0 < a , b < 1 0 , we have either a + b = 2 or a + b = 1 1 . The first case give the triplet ( 1 , 1 , 7 ) which does not satisfy third property.
The second case gives the following triplets:
( 2 , 9 , 7 )
( 4 , 7 , 7 )
( 6 , 5 , 7 )
( 8 , 3 , 7 )
Of which only first and third complies with third property, with first having the minimal product.
Hence the value: 126