Mixed Addition and Subtraction

Expressions that mix addition and subtraction appear in two shapes: add then subtract, and subtract then add. Even when the signs swap, as in 8 + 2 - 4, the work is done one step at a time from the left, and both Word Problems (with Pictures) and problems given in words alone confirm this.

The most frequent error is overlooking the sign and adding everything, giving 14 for 8 + 2 - 4. An answer equal to the sum of all three numbers identifies this pattern. In subtract then add, children keep subtracting through the second step as well, and the answer comes out too small.

This is the first unit where a plus sign and a minus sign sit inside the same expression. Reading the expression from its left-hand end still works exactly as before, and expressions with this same shape come around again in 3-Number Calculations in grade 2.