Although there are problems in the book and some extra drill exercises in class, one does not see any assessment that asks children to demonstrate mastery of all the sums to 18, nor is it clear that there are enough interspersed activities to practice these to mastery. The text clearly deals with how addition can be used to check subtraction. In fact, the relationship between addition and subtraction is stressed from the earliest lessons on addition and subtraction. This is an important topic to stress and master early. Because of the importance at this age of mastering addition and subtraction of multidigit numbers, a detailed description of the scope of adding such numbers follows.
Subtraction follows a parallel path, usually a chapter later.
Chapter 8 contains 8 lessons leading to the standard regrouping method for addition. This starts with stressing, using tens sticks and ones cubes, that adding 2 tens plus 3 tens is 5 tens, just as 2 ones plus 3 ones is 5 ones. This leads to adding multiples of 10 to any number mentally, by increasing the tens place accordingly. Regrouping is introduced in 8.6 via a game in which two one digit numbers, modeled with blocks, are added. If there are more than 10 blocks in the ones area, then ten are removed and a tens stick is added to the tens area.