Another maths question...

[Tomcat]

hi people...another maths question...can help to solve? thanks.

 

 

When Mrs Lee was 40 yrs old, her son was twice her daughter's age. Mrs Lee will be twice her son's age when her daughter is 28 yrs old. How old will Mrs Lee be when her daughter is 20 yrs old?

 

 

[Akari_ph]

let me guess, 80.

Edited June 7, 2008 by Akari_ph

[Scoots]

56

 

Let the son's age and duaghter's age when MRs Lee is 40 be x and y respectively.

 

x=2y

 

When Daughter reaches 28, Mrs Lee will be twice her son's age (note: number of years for Daughter's age to reach 28 =(28-y))

 

Mrs Lee's age = 2 x Son's age

40+(28-y)=2(x+(28-y))

68-y = 2(2y+28-y)

68-y = 2y+56

3y = 12

y = 4

 

Check condition: When daughter = 28yrs old, Mrs Lee age = 40+(28-4) = 64yrs old. Son's age = 8+(28-4)= 32yrs old (Mrs Lee's age = 2x Son's age - condition met)

 

Therefore, when daughter's age = 20 = 28-8, Mr's Lee's age = 64-8 = 56yrs old

Edited June 7, 2008 by Scoots

[Rcshark68]

yes... 56

[Arkantos]

ya i also get 56...

 

is that primary school maths?

looks quite hard....

[Tomcat]

yes, its a pri 6 maths question...think its too difficult for a kid at this level...got to show how to solve it in their method...

[Arkantos]

i use simultaneous equation leh...

think they will go lo. haha

 

i will work out using my method

use that as reference,

hope will be useful

 

L=Mrs lee B=Son G=Daughter

 

Certain year A L=40 B=2x G=x

Certain year B L=2y B=y G=28

To solve L=? B=NA G=20

 

let z be the number of years between year A and year B

 

40+z=2y

z=2y-40

 

2x+z=y

z=y-2x

 

x+z=28

z=28-x

 

comparing first and second

2y-40=y-2x

y=40-2x

 

 

comparing second and third

y-2x=28-x

y=28+x

 

compare again

40-2x=28+x

x=4

 

then can find y le

y=40-2(4)

y=32

 

 

so at year B Mrs lee is 64 years old and Daughter is 28 years old

when Daughter is 20 years old, Mrs lee will be 64-8=56 years old...

 

that question is

Edited June 7, 2008 by Arkantos

[Wind30]

Pri 6 learnt algebra but not simultaneous equation. by right should be able to solve this.

 

Let x=daughter's age when mom is 40.

 

from question, the kid should be able to form this single simple equation.

 

((28-x)+2x)*2=40+(28-x)

LHS is "son's age when daughter reaches 28" times 2 which is equal to moms age when daughters reaches 28.

 

solve for x

56+2x=68-x

3x=12

x=4.

 

when mom is 40, daughter is 4. The answer is trivial from this point onwards.

 

Can lah, primary 6 kids should be able to solve it if they can get the first equation. The trick is to help the kid formulate the first equation, with one single variable.

 

Anyone interested to get maths tuition for their kids??? :) my grades are like SUPER DUPER when I was in primary and secondary school :)

Edited June 7, 2008 by Wind30

[Albeniz]

I get 56.

 

The age difference between mother and son is always 32 (constant).

The age difference between mother and daughter is always 36 (constant).

The age difference between son and daughter is always 4 (constant).

 

When daughter is 4, son is 8 and mother is 40.

When daughter is 28, son is 32 and mother is 64.

When daughter is 20, mother will be 56 and son will be 24 (if you ask for son's age also).

[Wind30]

errr... I thought the hard part is getting the age difference.... you never show how to get the age difference.

[Albeniz]

Condition 1: When mother is 40, daughter's age is x and son's age is 2x.

Condition 2: When daughter is 28, son's age is y and mother's age is 2y.

 

Age difference between mother and son is always a constant = u.

Age difference between son and daughter is always a constant = v.

 

From condition 1 and 2: u = 40-2x = 2y-y -> 40-2x = y eqn (a)

v = 2x-x = y- 28 -> x = y-28 eqn (b)

 

Solving eqn (a) and eqn (b) x = 4, y = 32, u = 32, v = 4.

 

Hence when daughter is 20, mother is 20 + u + v = 56.

[Dynaway]

kill the school man n stop killing the kids.............. i dun even learn this when i was in primary school

 

 

crap... STOP KILLING THE KIDS....

[Guyver]

hi people...another maths question...can help to solve? thanks.

 

 

When Mrs Lee was 40 yrs old, her son was twice her daughter's age. Mrs Lee will be twice her son's age when her daughter is 28 yrs old. How old will Mrs Lee be when her daughter is 20 yrs old?

 

 

 

Wah .... chim.

 

Dear all, to solve this, cannot use simultaneous equation. Must use diagram.

[Wind30]

?? I actually did learnt such questions in Primary school 20+ years back. I think it is probably just one of the * questions which meant more difficult. If all the questions are easy, everyone will get full marks. Don't underestimate Primary 6 kids.

 

I mean I still remember some parents wrote to Straits times complaining how his top in class kid could not do PSLE Maths paper well and how it was too difficult for Primary 6 kids, etc, etc. Little did he know that many kids from the stronger schools probably aced the very same PSLE maths paper.

 

Seriously, the main issue with such question is framing the information in the question into one equation and then solving it.

 

I have given a solution which involves ONLY algebra learnt by primary 6 kids. No simultaneous equations.

[Mivec9]

Let x be age of daughter.

Age of Son = x + x = 2x

Age Difference between Son and Mrs Lee = 40 - 2x

 

When daughter is 28, Son's age is (28 + x)

Hence, Age of Mrs Lee = Son's age + Age Difference between Son and Mrs Lee

2(28 + x) = (28 + x) + (40 - 2x)

56 + 2x = 68 - x

2x + x = 68 - 56

3x = 12

x = 4

 

Daughter was 4 when Mrs Lee was 40. Age difference is 40 - 4 = 36 years old

If Daughter = 20, therefore Mrs Lee = 20 + 36 = 56 (ans)

 

Frankly, this question belongs to star question and is considered slightly too tough for average psle question. It is for those who want to score distinction

 

Hope this helps.

[Mivec9]

hmm think so far, think you have answered according to syllabus

[Mivec9]

I guess u were a top notch student then

[Mivec9]

i have friends whose kids didnt do well and many others were reported the maths papers were too tough. i find the last 2 years psle maths papers seem rather tough versus rest of years. I dun deny the ace schools have already drilled the kids on such level, so it is not surprising their students still can top the papers.

Edited June 7, 2008 by Xtraun