Discuss the solutions in the comment section.
# (10th grade math course) How many different strings of length 9 are there which contain 3 'a's, 3 'b's and 3 'c's.

# How many ways are there to climb a n level stair if at each step you can climb one or two levels.

# How many ways are there to climb a n level staircase if at each step you can climb one or two levels.

# (olimpiada online 2001) Given n points on a circle. Draw all possible n(n-1)/2 chords. What’s the maximum number of triangles one can see. Example: With n = 4 there are 8 triangles (4 with 3 of the 4 circle points and 4 with 2 circle points and 1 the intersection of the diagonal).
# ([4], agora scholarships 2001, 'infoarena':problema/drepte2) What is the maximum number of regions the plane can be split into by n lines. (Same problem for n circles, n planes, n spheres) Example: For n = 3 we can get 7 regions.
# ([1]) What is the maximum product for collection of natural numbers that sum up to n.