3
Answer: Two.....8-10 and 9-11
Answer: Any 2-digit number from 10 to 18 can be obtained by adding two 1-digit numbers.
9999
100,000
512
12
24
There are 504 such numbers.
There are 60480 numbers.
823543
6
840
There are 5! = 120 such numbers.
The number 1234567890 can create about 39 of 40 4 digit numbers.