|
|
|
| Formal written algorithms for subtraction | Decomposition algorithm | Decomposition using MAB | Equal addition algorithm | Equal addition using MAB | Quick Quiz |
In primary school children are taught a formal written algorithm for subtraction after they can subtract single digits from large numbers (e.g. by counting backwards or by mental subtraction) and can subtract multiples of ten (e.g. by counting backwards by tens). This means that they should be able to find 63 - 5 or 63 - 40 by informal means before learning a formal algorithm. They will probably also be able to find simple differences by counting forwards (e.g. to find that the difference between 18 and 30 is 12 because they can count 18, 19 , 20 and ten makes 30). The most commonly taught written procedures are the decomposition algorithm, and the equal addition algorithm. We will demonstrate the use of both algorithms to carry out the subtraction:
Before applying any algorithm it is a always a good idea to first estimate the difference as a rough check on the answer obtained by using the algorithm. Rough estimates can be found in many different ways. For example, one way would be to approximate this subtraction as 70 take away 40, so that the answer is about 30.
Click here to see a written description of decomposition Click on the movie below to see how decomposition is done: 
Click on the movies below to see other examples of decomposition:
Click on the movie below to see a variation on the decomposition algorithm:
Click here to see a written description of equal addition Click on the movie below to see the equal addition algorithm being used:
Click on the movies below to see other examples of equal addition algorithm:
Further questions: 1. Greg has 364 books. If Kara has 93 books less than Greg, how many books does Kara have? 2. Tess has 1398 stickers. How many more does she need to have 2520 stickers? 3. There are 160 cups in a restaurant and 37 get broken. How many are left? 4. Roslyn is 112 cm tall and Michael is 151 cm tall. What is the difference in their heights? 5. Paul has to run the 200 metre hurdles. He runs 137 metres and falls over. How many metres does he still have to run to finish the race? 6. The sum of two numbers is 159. If one of the numbers is 17, what is the other number? 7. Subtract 426 300 from 1092127 8. If 913000000 people live in India and 21000000 people live in Nepal how much bigger is India’s population than that of Nepal? To view the answers to the quiz, click here. If you would like to do some more questions, click here to go to the mixed operations quiz at the end of the division section.
|
|
| © University of Melbourne 2003 |
||
