A 5 Digit number

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)


The answer is 126.

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2 solutions

Mat Baluch
Jan 21, 2015

a b c a b = a × 1 0 4 + b × 1 0 3 + c × 1 0 2 + a × 10 + b abcab = a \times 10^{4} + b \times 10^{3} + c \times 10^{2} + a \times 10 + 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 ) 4a + 6b + 2c + 3a + b = 2c = 0 ( mod 7)

Since 0 < c < 10 0 < c < 10 we deduce c = 7 c = 7

From second property we deduce a + b + 7 = 0 ( m o d 9 ) a + b + 7 = 0 ( mod 9)

Since 0 < a , b < 10 0 < a,b < 10 , we have either a + b = 2 a + b = 2 or a + b = 11 a + b = 11 . The first case give the triplet ( 1 , 1 , 7 ) (1,1,7) which does not satisfy third property.

The second case gives the following triplets:

( 2 , 9 , 7 ) (2,9,7)

( 4 , 7 , 7 ) (4,7,7)

( 6 , 5 , 7 ) (6,5,7)

( 8 , 3 , 7 ) (8,3,7)

Of which only first and third complies with third property, with first having the minimal product.

Hence the value: 126

Awesome , I solved it using brute force and was surprised by your so easy solution

Ajinkya Shivashankar - 4 years, 7 months ago
Sai Krishna
Jan 19, 2015

divisibility rule for 4 is last two digits divisible by 4 and a,b,c>0 so this 18 possibilities.. and divisibility rule for 9 is sum of the digits is divisible by 9... so make a such that 18 possibilities are divisible by 9 and check them for 7

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