University of Cambridge
University of Cambridge: Nrich: The Amazing Splitting Plant
Figure out how the number of branches keeps doubling on this plant each week for six weeks. The solution is right there on this one page website.
University of Cambridge
University of Cambridge: Nrich: Skip Counting
Follow the clues Froggie gives you to help her skip count her way to the pumpkin patch. There are several solutions and you can check yours against the solutions right at this one page website.
Other
Math Stories: Write a Number Sentence
At this one page site you can work through step by step a sample problem that illustrates the problem solving strategy for "Write a number sentence." The answer is right there to help you check your thinking.