Q&A - PSLE Math

[tianzhu]

Lynn2:

In the recent exam, she has to do 18 times of guess and check just to get the answer.I think its quite a waste of time tho she eventually gets the answer.

Hi

You don’t really need to use GC for your question as the proportion of coins is given in the question. Solving it by grouping is much more efficient.
However, I’d use it as an illustration to show that a little bit of planning helps to save precious time in using GC.

In this particular question, there are 3 variables, 50 cents coin, 20 cents coin and 10 cents coins.

To get the right combination by getting the guessing randomly is extremely tough.

The question is structured in such a way that you can take away 90 cents (9 10 cents coin) from $11.30.This step is extremely important as it simplifies the solution greatly.

We have 2 times as many 50 cents coin as twenty cents coins. The number of ten cents coin is the same as the number of twenty cents coins after taking away 9.

Now group the ten cents and twenty cents together as a bundle of 30 cents. In this way, we only have two variables 50 cents and the newly formed bundle of 30 cents.

1130 - 90 ----- 1040

So we 1000 + 40 or 900 + 140 or 800 +240 (we are looking for multiples of 30)

240 ----- 8 groups of 30 cents, meaning 8 groups of 20 cents and 10 cents coins which translated into 8 20 cents coins and 17 ten cents coins.
For 800, you’ve 16 50 cents coins.

You need a bit of planning to save time when you use “GC”

Best wishes

[shurley197323]

Pls help. Siti opened a 2kg packet of flour. She used 4/9 of the flour to make a pizza. Then she used 2/7 of the remaining flour to make cakes. Find the mass of the packet of flour that she left. Express answer to 2 decimal place.

Thanks.

[jieheng]

The mass of the packet of flour that she left is


=5/9 * 5/7 * 2

=0.79 kg

[shurley197323]

jieheng:

The mass of the packet of flour that she left is


=5/9 * 5/7 * 2

=0.79 kg

Hi. Thanks. Any working please ?

[jieheng]

5/9 (the mass of flour she left after she use 4/9 for pizza)


5/7 (the mass of flour she left after she use 2/7 for cake)


The mass of the packet of flour that she left is

=5/9 * 5/7 * 2

=0.79 kg

[shurley197323]

Pls help.

1) Ms Lam wants to give some sweets to her pupils. If she gives 7 sweets to each pupil, she will have 380 sweets left. If she gives 15 sweets to each pupils ,she will have 76 sweets left.
(a) How many pupils are there ?
(b) How many sweets does Miss Lam have?

(Not allow to use guess & check methods)

(2) Sue bought an equal no of oranges & pears. Oranges cost 7 for $2 and pears cost 5 for $3. She paid $33 more for pears than oranges.
(a) How much did Sophia pay?
(b) How many oranges and pears did she buy altogether?

[jieheng]

Q1


a)

The difference between the sweets = 380 - 76 = 304

Extra sweets for each pupil = 15 - 7 = 8

No. of pupils = 304 / 8 = 38 (Ans)

b)


No. of sweets that Miss Lam has = 38 * 7 + 380 = 646 (Ans)

OR

No. of sweets that Miss Lam has = 38 * 15 + 76 = 646 (Ans)

[jieheng]

Q2


One set of equal no of oranges and pears = 7 * 5 = 35

Cost of 35 oranges = 35 / 7 * 2 = $ 10

Cost of 35 pears = 35 / 5 * 3 = $ 21

The difference between one set of fruits = 21 - 10 = $ 11

Given she paid $ 33 more for pears than oranges

No. of sets of fruits = 33 / 11 = 3

a) She paid = 3 * ( 10 + 21) = $ 93 (Ans)

b) No. of oranges and pears she bought = 3 * (35 + 35) = 210 (Ans)

[shurley197323]

Thanks jieheng. Don’t mind 1 more question.

2/5 of the counters in a box were red and tge rest were blue.
After putting 48 blue counters in the box, 3/4 of the counters were blue. How many counters were in the box at first?

[jieheng]

At First

Red Blue
2u 3u

+ 48counters

New Ratio 1u 3u
Change to 2u 6u
(There is no change in the no. of Red counters)


Different in the ratio of after and before for blue counters = 6u - 3u =3u
3u----->48

1u----->16

No. of counters at first = 5 u = 5 * 16 = 80 (Ans)