Fractions

Mixed numbers join in here, and calculations that treat the whole-number part and the fraction part separately begin. Once same-denominator addition and subtraction are confirmed through Calculation, Fill in the Blank and Word Problems, the work moves to subtraction With Borrowing, whole number minus fraction, whole number minus mixed number, and sums of three fractions.

When borrowing with mixed numbers, children take 1 from the whole-number part without adding the denominator to the numerator, so the numerator comes out short by exactly the denominator. For 3 minus 1 and 3/4, some subtract only the whole numbers and copy the fraction part straight into the answer.

The basic move of adding or subtracting fractions while the denominators stay fixed was already covered once in Fractions in grade 3. What is added here is the mixed number, a whole-number part fused to a fraction, and denominators keep matching until grade 5, when Properties of Fractions introduces the technique of finding a common one.