Multiply three digits by two digits and find the factor according to the result. The breakthrough of this problem is that the digit of 6 multiplied by 0 is 0, and there are 0 and 5. According to 6×7=42, this digit is 5, that is to say, the result is that the factor of 45 MINUS three digits is 5 of one digit. When the product of three digits is multiplied by 5, the highest digit of the factor of three digits is 1, so it can be solved.