Solve subtraction problems by using addition.
Number Framework Stage 6
Large Numeber Lines (Material Master 4-8)
In an example like this there is no need to jump up to 40 then to jump up to 79. This makes a simple problem harder. This is because the 7 in 37 is less than the 9 in 79.
Using Materials
Problem: Toni has $37 and is saving to buy a skateboard costing $79. How much does she need to save? Discuss the problem and write 37 + ? = 79 on the board. In pairs the students solve the problem with play money. (Or use the large class number line.) As a group discuss why the answer is 42. Two methods of solution are likely:
With money 3 tens and 4 tens make 7 tens - remember 4 tens. And 7 + 2 = 9. So the answer is 4 tens + 2 = 42.
With a number line 37 + 4 tens = 77, 77 + 2 = 79. So the answer is 40 + 2 = 42. Write 42 on the board.
Examples. 27 + ? = 69, 12 + ? = 67, 75 + ? = 95, 61 + ? = 83, ? + 26 = 96, ? + 26 = 96, ? + 51 = 82, ? + 36 = 59, ..
Using Imaging
Shielding: Problem: Shelley has $62 and needs a total of $89. How much does she need to save?
Write 62 + ? = 89 on the board. Model $62 (or use a number line) out of sight and ask the students to image what to do next. Record the answer. Fold back if necessary.
Examples 37 + ? = 79, 12 + ? = 69, 75 + ? = 95, 61 + ? = 83, ? + 36 = 77, ? + 23 = 49, ? + 68 = 98 …
Using Number Properties
Examples 17 + ? = 99, 5 + ? = 67, 15 + ? = 88, 4 + ? = 89, ? + 56 = 99, ? + 56 = 99, ? + 56 = 99 …
Challenging examples. 387 + ? = 399, 212 + ? = 777, 45 + ? = 795, 681 + ? = 999, ? + 256 = 759 …