Q&A - PSLE Math

[peggy]

Tianzhu,


thanks for replying with very clear illustrations.

[pixiedust]

I think similar questions have been discussed before. Please point me in the right direction. Thank you !!!


Mrs. Lim had some red and blue balloons in a bag. The number of red balloons was twice the number of blue balloons. She started removing balloons from the bag, each time taking out 4 red balloons and 6 blue balloons. After a while, only 120 red balloons were left in the bag. What was the total number of red and blue balloons in the bag at first ?

[tianzhu]

pixiedust:

I think similar questions have been discussed before. Please point me in the right direction. Thank you !!!


Mrs. Lim had some red and blue balloons in a bag. The number of red balloons was twice the number of blue balloons. She started removing balloons from the bag, each time taking out 4 red balloons and 6 blue balloons. After a while, only 120 red balloons were left in the bag. What was the total number of red and blue balloons in the bag at first ?

Hi

A neat way is to use MD, you may also use units.

Draw two boxes to show the number of red balloons and one box to show the number of blue balloons.

She started removing balloons from the bag, each time taking out 4 red balloons and 6 blue balloons.

Number of balloons removed is in the ratio of 2 red:3 blue (4:6 ----- 2:3)

Cut the blue box into 3 smaller units, Likewise, cut two units in one of the red box with the remainder showing 120.

Now do a comparison.

Red ----- 2 units + 120

Blue ------- 3 units

Remember, the number of red balloons was twice the number of blue balloons.

6 units ------ 2 units + 120

4 units ------ 120

1 unit ------- 30

Number of balloons@ first ------- 9 units ------ 270 (or 5 units + 120 ----- 270)

Best wishes

[jarenchuatw]

Thanks Master MathizzzFun.


Got one more question that is making my head crack…

Motorcyclist A and B started a 360km race. They rode their bikes at the same speed for the first 120km. For the rest of the 240km, A rode at a faster speed. He arrive at the finishing point 1omins before B who was 40 km behind him. B did not chang his speed for the whole race. B completed the rae at 3.10pm

a) At what time did they start the race?
b) What was A’s average speed for the last 240 km in m/min?

[MathIzzzFun]

jarenchuatw:

Thanks Master MathizzzFun.


Got one more question that is making my head crack....

Motorcyclist A and B started a 360km race. They rode their bikes at the same speed for the first 120km. For the rest of the 240km, A rode at a faster speed. He arrive at the finishing point 1omins before B who was 40 km behind him. B did not chang his speed for the whole race. B completed the rae at 3.10pm

a) At what time did they start the race?
b) What was A's average speed for the last 240 km in m/min?

hi

..me not Master la... just helping around here..

A reached the finishing point 10 min before B and B is 40 km behind
--> B will take 10 min to cover 40 km, ie B's speed = 40 km/10 min = 240 km/h
Total time taken for the 360 km = 360/240 h = 1.5 h = 1h 30 min
1h 30 min before 3.10pm --> 1.40 pm
They started the race at 1.40pm

For the last 240 km, when A covered 240 km, B covered only 200 km
So, A's speed = 240/200 x 240 km/h = 288 km/h

cheers.

[jarenchuatw]

Thanks MathIzzzFun for the prompt reply.


However, I could not quite understand part (b)…is there another way to explain it?

[MathIzzzFun]

jarenchuatw:

Thanks MathIzzzFun for the prompt reply.


However, I could not quite understand part (b)...is there another way to explain it?

Hi

for the last 240 km, time taken by B = 240/240 = 1h
Time taken by A = 1h - 10min = 50 min
A's speed = 240 km/50 min = 288 km/h

cheers.

[jarenchuatw]

MathsIzzzFun…wow, that’s so much clearer…million tks again!

[Essential]

Hi please help my DS for question 14 . John shifted the decimal point of a number twice to the left to obtain a new number . The difference between the new number and the original number was 136.62. A) how many times of the new number is the original number ? B) what is the sum of the 2 numbers ?

[MathIzzzFun]

Essential:

Hi please help my DS for question 14 . John shifted the decimal point of a number twice to the left to obtain a new number . The difference between the new number and the original number was 136.62. A) how many times of the new number is the original number ? B) what is the sum of the 2 numbers ?

Hi

decimal point shifted twice to the left = dividing by 100
so, new number is 1/100 of original number.
Difference = 99/100 of original number
original number = 136.62/99 x 100 = 138
.. with this, part b should be a breeze

cheers.