If length of string is single digit then it 's not divisible by 25
int n = str . length ( ) ; if ( n == 1 ) return false ; return ( ( str [ n - 1 ] - '0' == 0 && str [ n - 2 ] - '0' == 0 ) || ( ( str [ n - 2 ] - '0' ) * 10 + ( str [ n - 1 ] - '0' ) ) % 25 == 0 ) ; }