Calculations with 3 Numbers

Expressions with three numbers in a row, such as 3 + 2 + 4 and 9 - 2 - 3, are worked from left to right. Children find the middle result first and then add or subtract the next number, and follow the same order in Fill in the Blank and in Word Problems about passengers getting on and off.

For 9 - 2 - 3, computing only the first pair, writing 7 and forgetting the third number stands out. In addition, children lose track of the middle result and add only the last number to the first, so the middle number never reaches the answer.

Up through 2-Digit ± 1-Digit, every expression involved two numbers; the change here is that a third number joins them. Working through an expression from the left-hand end stays the guiding idea, and it carries straight into Mixed Addition and Subtraction, where plus and minus signs appear side by side.