These activities challenge children to solve all the addition and subtraction problem types they have already learned to solve and the very difficult Join and Separate start unknown problem type ("You had some cars and then you got 5 more, now you had 12. How many did you have to start with?"). Such activities benefit from discussions "parts" and "wholes." Children continue to build and use sophisticated  strategies, including seeing numbers as part of other numbers and using more well-known combinations (e.g., "5 and 5 is 10, so…").