As we get further, I take more things for granted and get into more difficult problems more quickly. In this section, you add three-digit numbers and can have up to four-digit sums. For these and similar problems, you may find that your memory has trouble keeping up with what's been added to what. For these problems, when I have trouble, I try to store no more than, say, four variables at a time in my head, and solve the problem by incrementally getting one number larger and the other one smaller. In any case, it will take time and practice. I don't say I've arrived myself.