[This is a slight plug, and I'm opening to deleting my answer.]
When you're looking for a site, your ability level would heavily influence the type of questions that you are able to answer. Without knowing much more about your knowledge and abilities, we cannot direct you to relevant examples. I looked at the questions you posed, and one of them was "How many ways to choose teams of 3-3-4 out of 10". Most people will consider this an extremely basic application of the binomial (or multinomial) theorem, as you can see from the answers that you get. However, you also asked questions on the Frobenius norm, and singular values of a matrix, so it's uncertain what the breadth and depth of your mathematical knowledge is.
I am the Mathematics Challenge Master on Brilliant.org and we provide weekly problem sets that are tailored to your ability level. Under the Geometry and Combinatorics topic (which includes probability), you will see several problems similar to the "choose teams" question that you posed. For the Algebra and Number Theory topic, some might be too simple given your knowledge, as we are mainly targeted at identifying talented high school students worldwide. We are planning on introducing the topic of linear algebra in the near future. Once again, there's a disclaimer that the problems might be too simple for you, given the kinds of questions that you are asking.
As a problem site, you will get 8 problems to work on every week. If you provide the correct numerical answer, there is a random chance that you will be asked to submit your solution. In the following week, student submitted solutions will be posted, so that you can learn from the different good approaches that were taken. In this way, you'd have sufficient time to work on the problems, without feeling forced to answer before someone else gets selected.
This is a problem taken from last week's Combinatorics set.