Addition And Subtraction

During Term 2 Room 12 have been working on different kinds of strategies to work out different kinds of equations. Each of the students have stepped up a level in Maths by learning all the strategies. So far we have learned five strategies. Here are the W.A.L.Ts to all the strategies.

Strategy 1: Use standard place value. Addition that adds up to nine.

Strategy 2: Use tidy numbers and compensate.

Strategy 3: Increase one number and decrease the other number by the same amount.

Strategy 4: Use standard place value partitoning (expanding) place value.

Strategy 5: Regroup to combine compatible numbers.

When we had finished completing worksheets that were based on these strategies our teacher got us to make-up our own word problems according to our topic about water:
Here is what I wrote as my word problem: I used strategy 3 to help me with my word problem.
S3 increase and decrease

Everyone in the Southern Hemisphere uses up to 15 litres of water to bathe and everyone in the Northern Hemisphere uses up to 45 litres of water to bathe.

How many litres in total does everyone around the world use? 

This is my caculation for the word problem:

15+5 + 45-5

=20 + 40

=60 litres of water