The Challenge Math is an online program offered by Noetic Learning. It is a very inexpensive way of receiving math problems for a specific grade level on a weekly basis. They offer 3 sessions, one in fall, one in spring and one in summer. The weekly assignments consist of 5 timed problems. It’s a good mix of problems touching topics like geometry, algebra, probability etc appropriate for the grade level. On an average the time commitment is 10-20 min per week. They have provided some samples questions on their website to try out before you register.