Discover Mathematics P4: Term 2, Unit 9, Lessons 5–7
GoalSubtract mixed numbers with the same denominator, regrouping a whole when needed.
New words
regroup a whole · to turn one whole into fractions so there are enough pieces to subtractdifference · what is left after subtractingerror analysis · finding and explaining a mistake in someone's work
Explanation
To subtract mixed numbers with the same denominator, subtract the fractions and subtract the wholes. For 3 5/8 − 1 2/8: fractions 5/8 − 2/8 = 3/8 and wholes 3 − 1 = 2, so the difference is 2 3/8. If the first fraction is smaller, we must regroup a whole. For 4 1/4 − 2 3/4, we cannot take 3/4 from 1/4. So change one whole of 4 into 4/4: 4 1/4 = 3 5/4. Now 5/4 − 3/4 = 2/4 and 3 − 2 = 1, so the difference is 1 2/4. Check by adding: 1 2/4 + 2 3/4 = 3 5/4 = 4 1/4. Never subtract the smaller fraction from the bigger one just to avoid regrouping.
A tank holds 5 litres of water and 2 3/5 litres are used. Find 5 − 2 3/5. Break one whole: 5 = 4 5/5. Then 5/5 − 3/5 = 2/5 and 4 − 2 = 2. The difference is 2 2/5 litres left. Check: 2 2/5 + 2 3/5 = 4 5/5 = 5.
A rope is 5 1/4 m long. Menna cuts off 2 3/4 m. 5 1/4 = 4 5/4. 5/4 − 3/4 = 2/4 and 4 − 2 = 2. 2 2/4 m of rope is left.
Class activity
Spot the error. A grown-up writes a subtraction of mixed numbers with a mistake in it (for example subtracting the smaller fraction from the bigger one). The child finds the mistake, explains it and corrects it.
