Auto comment: topic has been updated by cbdb (previous revision, new revision, compare).

It is not as hard as you think.
-The array is constructed using this formula b * i + c.
-An element of the array will be kept in the permutation if is in the range [0, n-1]
so for an element to be valid it should meet this condition :
b * i + c <= n-1.
so 0 <= i <= (n-1-c)/b, so valid numbers = (n-1-c)/b — 0 + 1.
-the remaining we need to add to permuatation will be n — valid ones, which is the answer to the problem

I'm not necessarily one to talk about this as I struggle with this myself, but I do suggest solving the 800-1000 rated problems in Codeforces as most of them have these types of problems, especially when tagged with "math."

For this particular problem, the algebra didn't trick me up as much as the casework for $$$b = 0$$$ is, but that's for another time. Also, I guess its good to make sure that you address possible floating point errors to avoid mistakes like this in the future.

Here's my code for reference (I used Python for big floats with relatively good enough precision)

startInd = ((n - c) / b) + 1
startInd = int(math.ceil(startInd))
startInd = max(startInd, 1)

i used bin search, i dont trust on my math skills