From site: On this website you will find thousands of our free mathematics enrichment materials (problems, articles and games) for teachers and learners from ages 5 to 19 years. All the resources are designed to develop subject knowledge, problem-solving and mathematical thinking skills. The website is updated with new material on the first day of every month.
LearningReviews.com note: As this site is hosted in the United Kingdom, the grade levels are shown as “stages.” Stage 1-2 problems would be for elementary age students in the US. Stage 2-3 problems would be for middle school students. Stage 3-4 corresponds to high school students. Stage 5 would be for AP high school students and college students.
Each problems gives you a link for hints to solve it. The solution link will not appear until the following month.