2
Counting on
Discover Mathematics P2: Term 1, Lessons 11–20
GoalAdd by counting on from the bigger number, and subtract by counting up.
New words
count on · to say the next numbers after the one you start withdifference · how far apart two numbers arenumber line · a line with numbers in order
Explanation
To add by counting on, start with the bigger number and count on the smaller one. For 3 + 12, say 12 and then count on 3: 13, 14, 15. So 3 + 12 = 15. You can use counting on to subtract too. For 8 − 6, start at 6 and count up to 8: 7, 8. That is 2 steps, so 8 − 6 = 2.
Worked examples
3 + 12: start at 12 and count on 3 steps: 13, 14, 15. The answer is 15.
10 + 6: start at 10 and count on 6 steps: 11, 12, 13, 14, 15, 16. The answer is 16.
8 − 6: start at 6 and count up to 8: 7, 8. That is 2 steps, so 8 − 6 = 2.
Class activity
Stair counting. Stand at the bottom of the stairs at home. Say a number like 5 and a number to add, like 3. Climb 3 steps while counting on: 6, 7, 8. Try subtraction by climbing up from the small number.
Practice
1
Count on from 9 to find 9 + 4.
13 (10, 11, 12, 13)
2
Find 2 + 14. Start with the bigger number.
16 (14, then 15, 16)
3
Find 11 − 9 by counting up from 9.
2 (10, 11)
4
Find 17 − 14 by counting up from 14.
3 (15, 16, 17)
5
Karim has 12 marbles. He wins 4 more. How many marbles does he have now?
16 (13, 14, 15, 16)