It is good that you help to solve the problem sums for PSLE
students.
But you need to solve the problem sums in a way that PSLE
students can understand (model drawing method), not to solve the
problem sums using the method that you are familiar with (secondary
school algebra).
You have told dkcx "in the thread Primary 6 Maths" that you
NEVER teach your students to try and guess the correct answer for
problem sums but you have just used it for your solution for
question two.
" By trial and error, we let y be 2 and x be 1,
5(2) - 4(1) = 2 ( rejected
)
Now, we let y be 4 and x be
2,
5(4) - 4(2) = 4 (
correct answer )"
You are contradicting yourself. It is also NOT true that guess
and check method cannot be used in PSLE exam as you have claimed
it. The guess and check method can be used in PSLE exam but it is
usually used only as a last resort.
For your information, question 2 is a variation of the "Money
not Enough" type of question (ie one of the many
PSLE questions classified under the different patterns and
topics) in PSLE exam.
Steps to solve question 2 using the model drawing
method.
1. $2.50 = Five 50-cent
coins $1.50 = Three 50-cent
coins
$4 = Eight 50-cent
coins $3.50 =
Seven 50-cent coins
2. Since the amount of the money must be the same for both
cases, we will draw two equal length bars.
3. First
Bar : 5 : 3
:
:
Second
Bar : 8
:
7 :
:
4. Next, we will add 8 into the empty space in the
second bar
First Bar
: 5 : 3
:
:
Second
Bar : 8
:
7 : 8
:
5 7 + 8 = 15
since 15 can be divided by 5, we will
not add more 8s into the empty space in the second bar.
6. Hence,
First
Bar : 5 : 3
: 5 : 5 :
5
:
Second
Bar : 8
:
7 : 8
:
7. So, the minimum amount of money she has = (8 + 7 +
8)coins x 50 cents
= $11.50
Alternatively, the minimum amount
of money she has
= (5 + 3 + 5 + 5 + 5) coins x 50 cents
= $11.50
My method here is a set way of trial and error with proper
workings , what I am against is random guessing of answers (
tikum-tikum ).
Even for guess and check questions, there is usually a standard
set of base workings, without which you cannot solve the
problem.
Once again, I will have to argue that only Q2. is worked out in
a way not suitable for Pri school students, though I believe that I
stated I have no other choice since I cannot draw out models.
On the otherhand, I will have to thank you for showing me a
method of 'drawing' without using actual models. I hope you will
not mind if I borrow your method to 'draw' models for others to
see.