Guess Digits Sum game

Can you guess the sum of K digits fast enough?

Given

Let number be K digits distributed uniformly (K=4: numbers are 0000, 0001, ..., 9998, 9999).
Let n be the sum of digits in the number (K=6: n(098706)=30).
Let guess be a single attempt to guess n. If you miss, you get a hint if your guess is too high (guess > n) or too low (guess < n).

Required

The Solution (the algorithm) which finds n in the least number of guess attempts.

Questions (in order of complexity)

1. A, B, C (first three guesses for K=4)

Having The Solution and K=4, your first guess is A(get n < guess), second one is B (get n < guess) and third one is C. Tell A, B, C.

2. Total number of guesses for K = 8

Assuming g(number) is the number of guesses it takes your algorithm to find the n for number, find the average number of guesses in K = 8 range (00000000 to 99999999). To find this, simply sum g(number) for all numbers in range (and divide by their count).

Example answer: total number of guesses is 423297777 or 4.23297777 guesses on average.

3. Total number of guesses for K = 16, 20

Having The Well Coded Solution, provide Question 2 answer for K = 16 and K = 20.

Example answers:

K = 16, number of guesses is 46763514977777777 or 4.6763514977777777 guesses on average
K = 20, number of guesses is 484743581777777777777 or 4.84743581777777777777 guesses on average

4. O(K) *

Find Computational complexity in big O notation for The Solution.

* I don't know the answer, maybe you do :)

Don't read this

Some stuff is available by

$ ruby g_sum.rb 6

The Well Coded Solution will be provided after the contest.

p1366/guess_digits_sum