ABCD x 4 = DCBA

ABCD x 4 = DCBA.

What are the values of A, B, C and D? (Write the answer as a whole number in ABCD order, eg if A=3, B=9, C=8, D=6 write "3986")


The answer is 2178.

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1 solution

Jerry McKenzie
Jan 27, 2018

First note in the thousands place, A A must be 1 or 2. Then note that since A A will also be in the ones place in the product with 4, A A must be even and must be 2. It then follows D = 4 A = 8 D=4A=8 since 8 4 = 32 8\cdot 4=32 and 2 is in the ones place as desired.

Then we can see that the numbers B C BC follow the pattern 4 C + 3 = 10 x + B 4C+3=10x+B , where x x is any amount carried and 4 B + x = C 4B+x=C . Noting that 4B cannot carry to the thousands place B 2 B\leq 2 . Looking at 4 C + 3 = 10 x + B 4C+3=10x+B , 4 C + 3 4C+3 must be odd, so B B must be 1 (since 10x is even). Lastly we can solve the system 4 C + 3 = 10 x + 1 4C+3=10x+1 and 4 + x = C 4+x=C and find C = 7 C=7

Thus the required answer is 2178.

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