# BAR MODEL METHOD in PSLE Math

Ho Soo Thong

First Edition, Nov 2015 ; Revised in July 2016; April 2018

# BAR MODEL METHOD in PSLE MATH

** PSLE Math – A Bar Modelling Problem Solving Approach**

Prior to 1980, some school teachers in Singapore hit upon the idea of using bars instead of line segments to represent mathematical quantities. They adopted a heuristic approach by applying basic mathematics to construct bar models to depict and compose mathematical situations in word problems.Finally we use the *Part-Whole* and *Comparison* Concepts and apply simple Algebra for a complete solution. The modelling approach provides a visual aid for understanding and solving the problems.and is known as the ** Bar Model Method**.

### Bar Model Method Involves Two Major Steps:

1. Understand *Mathematics Situations* and construct *Bar Model* to visualise the problem.

2. Apply simple *Arithmetic Operations* and look for *Algebraic Relations* on bar models for unknown situations.

**For Modelling “Division” Situations – Euclidean Division Algorithm **

We begin with a counting problem with mathematical linear expressions.

**Example 1** *A Sweet Problem*

Jane, a teacher, wanted to give each of her 46 pupils a package of 6 sweets. The sweets were bought in boxes of 20. How many boxes were needed?

Solution

The bar model shows that Jane gave away 46*×*6 = 276 sweets and Jane needed 13 + 1 = 14 boxes.

Notes : The example illustrates the repeated use of of *Euclidean Division Algorithm* to depict two division situations.You may visit a related problem (Click The Ribbon Problem ),a variant of 2017 PSLE question where the remainder is unusable in contrast to the above problem. The two problems illustrate “Remainder” situations with usable or unusable remaining items in some word problems. For more complex problems, you may refer to the book “*Bar Model Method for Job Problems*“

**For Distributive Property Situations – Mathematical Deduction**

The next problem involves a problem solving strategy for distributive problems.

**Example 2*** – An Apple-Orange Distributive Problem *

Kim paid $ 6.60 for 5 apples and 2 oranges. Ann paid $ 6.00 for 2 apples and 5 oranges.

(a) Lee bought 4 apples and 3 oranges, how much did Lee pay?

(b) How much did Mei pay for 7 Apples ?

Solution

First, we construct bar models to visualise the difference between Kim’s situation and Ann’s situations:Kim has three more apples and Ann has three more orangesand deduce that an apple cost $0.20 more than an orange.

Notes :

- The broken bar model of Ann’s situation is constructed in such a way that we can compare it visually with the bar model for Kim’s situation.
- The applications of
*Distributive Law*can be seen in*Excess-Shortage Problems*and variants of*Chicken-Rabbit Problems*.

**For “Multiple” Situations – ***Unitary Method*

*Unitary Method*

We begin the use of mathematical symbol U and simple algebra for a counting problem.

**Example 3** *A Changing Weight Problem*

There are two bags of sugar, *X* and *Y*. If the weight of bag *X* is increased by 3 kg, bag *X* will be 2 times the weight of bag *Y*. If the weight of bag *X* is decreased by 8 kg, bag *Y* will be 3 times the weight of bag *X*.

What are the original weights of each bags of sugar?

*Solution*

There are three situations.

We begin the “Before” unknown situations and then the two “after” situations. Next, we identify the common unit of *U *kg * *and proceed with simple Algebra.

Note : We use the simple algebraic expression 3*U* for “Multiple” situations.

For “Multiple” Situations, we begin with the concept of unit with a mathematical symbol U for algebraic expressions, simple algebraic relations and simple algebraic manipulations to solve simple algebraic equations – *Linear Equations*.

**For Ratio Situations - A Ratio Approach with Unitary Method**

The next problem involves the property of divisibility by two numbers.

**Example 4** *A Boy-Girl Ratio Problem*

The ratio of the number of boys to the number of girls in a social club was initially 5 : 2. When 14 new girls joined the club, the ratio became 4 : 3. How many members were there at first?

*Solution** *

By noting that the number of boys remained unchanged, we construct the bar models for the *Before* and *After* situations

*Before* situation* : *Number of boys : Number of girl = 5 : 2.

*After* situation* : *Number of boys : Number of girl = 4 : 3.

Note : The *Ratio Approach* is an effective method for many PSLE questions involving ratios, fractions and simple percentages.

**For Algebraic Variations on Bar Models – Simple Algebra**

The following example illustrates a problem strategy involving varying values of two mathematics quantities values. The bar modelling approach can be applied to a recent PSLE question(05/02/2018).

.

**Example 5 –*** A Stamp Collection Problem*

Ann and Tim both have stamp collections. At first, Ann had 7 more stamps than Tim. After Ann giving Tim 9 stamps, how many more stamps then Ann did Tim have?

*Solution*

We divide Ann’s activity of giving 9 stamps in two steps:

Step 1 : Ann gave 7 stamps to Tim

Step 2 : Ann gave another 2 stamps to Tim

A visual view of a bar modelling problem solving strategy involving changing an “equal parts” and a “more than” part as shown in the following bar models.

Notes : The above illustrates a heuristic approach to an algebraic relation on the bar models. We could use bar models to visualise variables with complex algebraic relations and suggest the further use of bar models for more advanced problems involving complex algebraic relations.The above example illustrates the visual algebraic operations on the bar models.

**For Property of Relatively Prime Numbers.**

Finally, we reach a higher level of mathematics to show how the use of the property of *relatively prime numbers *for a counting approach to a challenging problem. Pupils can use the mathematics to solve a recent PSLE question (17/02/2018).

**Example 6 – ***A cupcake Problem*

May baked more than 100 cupcakes and she packed all of them into some small and large boxes. Each large box contains 8 cupcakes and each small box contains 5 cupcakes.

The total of cupcakes in the large boxes is 13 more than that in the small boxes.What is the least number of cupcakes she baked?

*Solution*

First, we construct a bar model for the situations as shown.

Since 5 and 8 are relatively prime numbers and the least common multiple of 5 and 8 is 5•8 = 40. Now

The least number of cupcakes is 163.

* More examples* can be seen in the site alpha-psle.com.