Problem
In my backyard there is a tree. The tree can hold 20 birds. When I looked outside I saw 15 birds sitting in the tree. 12 birds are flying towards the tree. How many of these new birds can land in the tree? How many of these new birds will have to fly away?

What is this problem about?
This problem involves the children in a series of simple additions and subtractions.

Achievement Objectives
Number (Level 2)
- write and solve story problems which involve whole numbers and which require a choice of any combination of the four arithmetic operations.

Mathematical Processes
- devise and use problem solving strategies (act it out, draw a picture, use equipment).

Resources
Paper for drawing a tree and counters to use as model birds, or paper and pencils.
Blackline master of the problem (English)

Specific Learning Outcomes
The children will be able to:
- use combinations of addition and subtraction to solve the problem
- skip count in fives
- investigate a problem and use problem solving strategies;

1. Introduce the problem by discussing how many birds could "fit" in a tree.
2. Read the problem to the class.
3. Ask the children to identify the important information given in the problem and highlight it.
4. Brain storm for ways to solve the problem.
5. As the children work ask questions that focus their thinking on the number operations that they are using:
   What are you doing to solve the problem? Why did you choose that?
   How did you add the numbers together?
   How did you subtract the numbers? Why did you subtract those numbers?
6. Share the solutions or by using any other method they suggest

Other Contexts
Any situation where there are a fixed number of resources can be used. For instance, pegs for children to put their clothes on.

Solution
If the children use the counter method suggested above, then it might be worthwhile for them to first put the 15 green counters to represent the 15 birds in the tree. Then they might draw circles for the spaces that are still left for the birds. This can be done by counting on from 15 to 20. If there are 12 red counters to represent the new birds that are flying toward the tree, the children can put these one by one onto the circles until all the circles (and therefore all the spaces) are used up.

By counting all the red counters that are now in the tree (or by counting the number of circles they drew), they will see that 5 birds can land in the tree.