Open sentences such as 21 + N = 59, 9 + N = 17, 18 − N = 9, and so on, occur regularly in elementary school mathematics textbooks. Children learn to solve sentences such as 9 + 6 = N and 38 − 19 = N, where the placeholder is by itself on one side of the equality sign, much earlier than open sentences where the placeholder is in a different position such as 14 − N = 8 or N + 17 = 56. Children's performance is not easily predicted when open sentences of the latter forms are involved. For example, it is not clear which, if any, of the open sentences 24 + N = 58, N + 24 = 58, 58 − N = 24, or N − 24 = 34 children have the most difficulty solving. Which would you predict?