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")
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First note in the thousands place, A must be 1 or 2. Then note that since A will also be in the ones place in the product with 4, A must be even and must be 2. It then follows D = 4 A = 8 since 8 ⋅ 4 = 3 2 and 2 is in the ones place as desired.
Then we can see that the numbers B C follow the pattern 4 C + 3 = 1 0 x + B , where x is any amount carried and 4 B + x = C . Noting that 4B cannot carry to the thousands place B ≤ 2 . Looking at 4 C + 3 = 1 0 x + B , 4 C + 3 must be odd, so B must be 1 (since 10x is even). Lastly we can solve the system 4 C + 3 = 1 0 x + 1 and 4 + x = C and find C = 7
Thus the required answer is 2178.