Change start-unknown problems to change-unknown problems.
Solve addition and subtraction problems by compensating with tidy numbers.
Number Framework Stage 5
Number Lines (Material Master 5-12)
This learning experience involves solving a start unknown problem. The student develops the understanding that the problem can be solved by changing the order of the addends (commutative property): ? + 29 = 81 to 29 + ? = 81. (If they do not understand this idea initially, work with smaller numbers.) Problems should be presented as word problems using the start unknown structure, not as equations. The students should solve them using an appropriate mental strategy.
Using number properties
Problem: Taine has some cows. Taine buys twenty-nine more cows. Now he has eighty-one cows. How many cows did Taine have before?
Here the unknown number comes first. Discuss why this may be written as ? + 29 = 81. Discuss reversing the problem to 29 + ? = 81 and solve it. Students can solve these problems by choosing wisely from the different methods they have learned.
Examples: Word problems and recording for: ? + 38 = 72, ? + 33 = 83, ? + 18 = 43, ? + 9 = 62, ? + 23 = 72, ? + 21 = 88, ? + 25 = 70 …