Knowing the sum of doubles, like 8 plus 8, and learning to skip-count (count every other number) is another shortcut. Many teachers encourage 1st graders to memorize their doubles up to 20, and to be able to count by 2's, 5's, and 10's to 100. For example, if your child knows instinctively that 8 plus 8 is 16, he simply has to add one to know the answer to 8 plus 9.