Discover Mathematics P4: Term 2, Unit 9, Lessons 5–7
GoalAdd mixed numbers with the same denominator and regroup when the fractions make a whole or more.
New words
regroup · to turn a fraction that is 1 or more into a whole number and a fractionmixed number · a whole number and a fraction togetherlike denominators · denominators that are the same
Explanation
To add mixed numbers with the same denominator, add the whole numbers and add the fractions separately. For 2 1/4 + 1 2/4: the wholes give 2 + 1 = 3 and the fractions give 1/4 + 2/4 = 3/4, so the sum is 3 3/4. Sometimes the fractions add up to more than a whole. For 1 3/5 + 2 4/5 the wholes give 3 and the fractions give 7/5. Because 7/5 = 5/5 + 2/5 = 1 2/5, we regroup: 3 + 1 2/5 = 4 2/5. We can also change both numbers into fractions: 8/5 + 14/5 = 22/5 = 4 2/5. Never add the denominators: five fifths plus five fifths is ten fifths, not ten tenths.
1 3/5 + 2 4/5 = 4 2/5
Step
Work
1
Add the wholes: 1 + 2 = 3
2
Add the fractions: 3/5 + 4/5 = 7/5
3
Regroup: 7/5 = 1 2/5
4
Total: 3 + 1 2/5 = 4 2/5
Worked examples
Find 2 1/4 + 1 2/4. Wholes: 2 + 1 = 3. Fractions: 1/4 + 2/4 = 3/4. The sum is 3 3/4.
Marwan walks 1 5/8 km in the morning and 2 7/8 km in the afternoon. Wholes: 3. Fractions: 5/8 + 7/8 = 12/8 = 1 4/8. Total: 3 + 1 4/8 = 4 4/8 km.
Class activity
Add by measuring. Measure two pieces of string, each a little more than 1 and a half hands long, marked in halves or quarters. Add the lengths as mixed numbers and check by laying the pieces end to end.
