Mental computation = "Mentally estimates and calculates addition and subtraction to 100 strategies based on ones and knowledge of number facts" (Bobis, Mulligan & Lowrie, 2008, p.220).
There are several models/representations of mental computations of addition and subtraction strategies which are:-
- Tens frame
- Hundred charts
- Empty number lines
Tens frame
Two different ways of showing 8:-
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Addition by using tens frame:- 7 + 5
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| Fill in the tens frame with blue counters, thus 7 + 5 = 10 + 2 = 12 |
Hundreds chart
"The hundreds chart is a well-known representation of numbers 1-100 (or 0-99) that has a multitude of uses for developing counting, patterning, base ten, addition and subtraction, and multiplication and division fact knowledge. The use of partitioning and sequencing ('split and jump') methods can be assisted through the complementary use of the hundreds chart" (Bobis, Mulligan & Lowrie, 2008, p.223).
Addition by using hundreds chart:-
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| 57 + 24 = 81 (Put the purple counter at 57) 57 + 20= 77 (Put the orange counter at 77) 77 + 4 = 81 (Move the orange counter to 81) |
Empty number lines
Empty number lines is an effective representation for addition as the children can do the calculation by the 'split and jump' method in the number sequence.
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| 'split and jump' method for addition |
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| 'split and jump' method subtraction |
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| alternative 'split and jump' method for addition |
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| alternative 'split and jump' method for subtraction |
Teacher could use these models/representations in order to support children's mental computation development of addition and subtraction strategies effectively and efficiently.
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