3
Adding four numbers
Discover Mathematics P2: Term 1, Lessons 31–40
GoalAdd four numbers by making two pairs, adding each pair, and then adding the two totals.
New words
pair · two numbers we add firsttotal · the answer after addingadd on · put one number after another
Explanation
You can add four numbers two at a time. Make two pairs. Add the first pair. Add the second pair. Then add the two totals. For 12 + 18 + 21 + 19: 12 + 18 = 30 and 21 + 19 = 40, then 30 + 40 = 70. Look for pairs whose ones make 10, such as 2 and 8 or 1 and 9. They are easy to add.
Worked examples
15 + 25 + 30 + 14. Pair 15 + 25 = 40 and 30 + 14 = 44. Then 40 + 44 = 84.
| Pair | Total |
|---|---|
| 26 + 14 | 40 |
| 33 + 27 | 60 |
| 40 + 60 | 100 |
Pairs can be any two numbers. For 45 + 13 + 27 + 12 do 45 + 13 = 58 and 27 + 12 = 39. Then 58 + 39 = 97.
Class activity
Pairing cards. The child picks four cards from a pack with the numbers 10 to 30 written on them. Together find the two best pairs, add each pair, and add the totals. Try a different pairing and check that the total stays the same.
Practice
1
Add 11 + 19 + 23 + 17 in pairs.
11 + 19 = 30 and 23 + 17 = 40. Total 70.
2
Add 34 + 16 + 25 + 15.
34 + 16 = 50 and 25 + 15 = 40. Total 90.
3
Nour counted 22 red, 18 yellow, 31 white and 19 pink flowers in the garden. How many flowers are there in all?
22 + 18 = 40 and 31 + 19 = 50. There are 90 flowers.
4
Pair the numbers in the table so that the ones make ten, then add: 14, 27, 16, 13.
| A | B | C | D |
|---|---|---|---|
| 14 | 27 | 16 | 13 |
14 + 16 = 30 and 27 + 13 = 40. Total 70.
5
Add 25 + 32 + 15 + 18 in the best way you can.
25 + 15 = 40 and 32 + 18 = 50. Total 90.