Solution: We assume that the interest in mathematics is expressed by ABCD. The first step analysis: ABCD multiplied by 4 is still four digits. How can it be deduced? A must be 1 or 2, because it will be carried if it is greater than 2. Where a is 1 or 2; The second step is analysis: the mantissa of the number after multiplying D by 4 is equal to A. We can imagine that from 0 to 9, the mantissa of the number after multiplying 4 is not equal to 1, so according to the previous step, it can be concluded that A can only be equal to 2. The third step is analysis: A times 4 equals D, which may be taken back. Considering the possibility of carrying, which D is equal to 8 or 9, let's take it back and find the hypothetical D, which D can only be 8. Step 4 Analysis: If D is equal to 8, then there is no carry after B is multiplied by 4. If you carry it, it will be greater than 8, right? B can only be 1 or 2, but ABCD is four different numbers, so B can only be 1. Step 5 Analysis: Now we know that it is not difficult to calculate C when A = 2, B = 1 and D = 8. Or 1C8*4=C 12. Let's analyze, 4*8=32 is equivalent to inputting a number 3, that is, the mantissa of the number after C multiplied by 4 plus 3 is equal to 1. Which mantissa is equal to 8 when C is multiplied by 4, and which mantissa is equal to 8, 2 or 7 when 4 is multiplied by 4, because it can't be 2 (A is equal to 2), which one can only be that C is equal to 7. ABCD=2 178