## Realated Questions

Find the number of pairs of positive integers (a, b) such that a!+b! = a

^{b}What is the remainder when 21000 is divided by 25?

In a four-digit number, the sum of the first two digits is four-fifth of the sum of the last two digits, while the sum of the first and last digits equals the sum of the other two digits.If the first digit is less than the second digit, how many such four-digit numbers exist?