Q&A - PSLE Math

[tianzhu]

Michaelia0816:

May I knew the heuristic way to solve this kind of question,


There were 160 boys and 100 girls at the Christmas party. An equal number of boys and girls left the party before its ended. In the end, the number of boys at the party became thrice the number of girls. How many girls left the party?

Hi

Since an equal number of boys and girls left the party, the difference in the number of boys and girls remains the same before and after.

You may use MD or UM.

In the end

Boys:Girls ------- 3:1

2 units ------- 60 (160-100)

1 unit ------- 30

Number of girls who left the party ------ 100 - 30 ------ 70

Best wishes

[Michaelia0816]

How to do the following questions?

I got confused with some word in, pls give me the ful solutions,

1) There were 160 boys and 100 girls at the Christmas party. An equal number of boys and girls left the party before its ended. In the end, the number of boys at the party became thrice the number of girls. How many girls left the party?

2) Mr Toh is 28 years older than his daughter. How old will his daughter be when he is thrice as old as she is?

3) Mary is 5 years older than Alice. In 3 years’ time, Mary will be twice as old as Alice. How old will Mary be in 10 years’ time.

I got a lot of similar question like Q1 in my WS, so of I dun understand, I may ask

[tianzhu]

Hi girl


I’ll further elaborate on my earlier solution.Hope this helps.

At the beginning of the party we have 160 boys and 100 girls. This gives a difference of 60 more boys.

In the end, the number of boys at the party became thrice the number of girls.

Boys:Girls ------- 3:1

Since an equal number of boys and girls left the party, the difference in the number of boys and girls remains the same before and after.

2 units ------- 60 (160-100)

1 unit ------- 30

Number of girls who left the party ------ 100 - 30 ------ 70

Best wishes

PS ----- It’s important to understand the concept and be able to apply to similar types of questions during examinations. Try going through the solution again and if it bothers you, let me know which areas tripped you. I’ll try to see if I can make it clearer.


Flow of events

Number of boys@first ------- 160
Number of girls@first ------- 100
Difference@first ------- 60

Number of boys@left the party ------- 70
Number of girls@left the party ------- 70

Number of boys@end------- 90 (160-70)
Number of girls@end ------- 30 (100-70)
Difference@first ------- 60

[tianzhu]

Michaelia0816:

2) Mr Toh is 28 years older than his daughter. How old will his daughter be when he is thrice as old as she is?

Hi

Good Morning.

In doing “Age question”, one needs to understand that the difference in age between two persons will always be the same.

Take for example, if your mother is 28 years older than you are, forever she’ll be 28 years older than you.

You may use MD or UM.

Draw 3 boxes to show Mr Toh’s age.

Draw 1 box to show his daughter’s age.

Their difference in age is shown by 2 boxes which is 28.

2 units ------ 28

1 unit ------ 14

His daughter will be 14 years old.

Best wishes

[tianzhu]

Michaelia0816:

3) Mary is 5 years older than Alice. In 3 years' time, Mary will be twice as old as Alice. How old will Mary be in 10 years' time.

Hi

Again, rely on the key point “Age difference stays the same”.

I believe the best way is to let you try out the question yourself.

Here are some pointers; you need to find their present ages first (Alice --- 2 and Mary --- 7)

The answer is 17.

Best wishes

[Michaelia0816]

Ok , I almost needed to use GC, But my teacher told me in class yesterday that got a heuristic way!

[Michaelia0816]

Thx Tianzhu!

[Michaelia0816]

For my Q2, my statement is the same bit I did not stop at 14,I times it, my big mistake!

[Michaelia0816]

tianzhu:

Hi girl


I’ll further elaborate on my earlier solution.Hope this helps.

At the beginning of the party we have 160 boys and 100 girls. This gives a difference of 60 more boys.

In the end, the number of boys at the party became thrice the number of girls.

Boys:Girls ------- 3:1

Since an equal number of boys and girls left the party, the difference in the number of boys and girls remains the same before and after.

2u-------60
1u-------30

Number of girls who left the party ------ 100 - 30 ------ 70

Best wishes

PS ----- It’s important to understand the concept and be able to apply to similar types of questions during examinations. Try going through the solution again and if it bothers you, let me know which areas tripped you. I’ll try to see if I can make it clearer.


Flow of events

Number of boys@first ------- 160
Number of girls@first ------- 100
Difference@first ------- 60

Number of boys@left the party ------- 70
Number of girls@left the party ------- 70

Number of boys@end------- 90 (160-70)
Number of girls@end ------- 30 (100-70)
Difference@first ------- 60

Why must be divided by 2? I tried to divide it but cannot get the answer, I was struck at this statement at my HW.

[tianzhu]

Michaelia0816:

Why must be divided by 2? I tried to divide it but cannot get the answer, I was struck at this statement at my HW.

Hi

In the end, the number of boys at the party became thrice the number of girls.

Boys:Girls ------- 3:1

In MD format, we’ll have 3 boxes to show the number of boys and 1 box to show the number of girls.

The difference is 2 boxes or units.

2 units ------ 60

1 unit ------ 30

Best wishes