Q&A - PSLE Math

[Dazzle]

Pls help to solve this St. Hilda Prelim Q15

http://i50.tinypic.com/2zyabr8.jpg\">

[Dazzle]

http://i45.tinypic.com/2dvralg.jpg\"> Realised the earlier pic is way too small. see if this is big enough

[MathIzzzFun]

ksmama:

[quote]

MathIzzzFun:

After completing 2/7 of the journey in 1/2 hour, the car passed a van travelling at a constant speed of 70km/h in the same direction.
3 hours later, the car reached Town Q but the van was still 65 km away from Town Q.

(a) Find the average speed of the car.

(b) If the van left Town P at 0810, at what time would it arrive at Town Q?



5/7 of distance PQ --> 3 x 70 + 65 = 275 km
distance PQ --> 275/5 x 7 --> 385 km
total time taken by car --> 3. 5 h
average speed of car --> 385/3. 5 --> 110 km/h

cheers.

Hi mathsizzfun,
Regardg yr ans to the foll questn, my kid calculated a diff way n the ans is different. Please comment on why his method is not acceptable. Is it because when d ques asks for average sp, we have to calculate based on whole dist n time taken n not assume aver sp same throughout? Thanks.

Diff in speed-> 65/3 =21.67km/h
Speed of car 70+21.67 = 91.67km/h.

[/quote]It is incorrect to find average speed by finding the average of averages ie average = 1/2 x (average 1 + average 2)

The definition of average speed is
Average speed = (total distance travelled)/(Total time taken)

For eg. Mr Tan travelled 20 km at 40km/h and another 20 km at 60km/h, what is his average speed ?
It is incorrect to calculate average speed = 1/2 x (40+60)

The right way is to find the total time taken --> 20/40 h + 20/60 h = 5/6 h
average speed = (total distance travelled)/(total time taken) = 40 ÷ 5/6 = 48 km/h

cheers.

[tianzhu]

Dazzle:

http://i45.tinypic.com/2dvralg.jpg\"> Realised the earlier pic is way too small. see if this is big enough

Hi

When dealing with composite figures, try as much as possible to deal with familiar shapes. At this moment, just days before PSLE, calculating the areas of familiar shapes should be at students’ finger tips.

Area of quadrant (r10) – area of isosceles triangle (1/2*10*10) --------- Area of half a leaf or rugby ball (Area A)

Area A - area of triangle (1/2*5*5) ------ Difference in X - Y

I'll leave your child to do the calculations.

Best wishes

[MathIzzzFun]

Dazzle:

http://i45.tinypic.com/2dvralg.jpg\"> Realised the earlier pic is way too small. see if this is big enough

http://i45.tinypic.com/29pra5u.png\">

cheers.

[Essential]

Can help this question urgently?

Beatrice has some twenty-cent and fifty-cent coins. 2/5 of the total value of the twenty-cent coins is the same as 1/2 of the total value of the fifty-cent coins. If Beatrice spends 4 fifty-cent coins, the value of the twenty-cent coins and the value of the remaining fifty-cent coins will be the same. How many twenty-cent coins does Beatrice have? Ans 30.

[tianzhu]

Essential:

Can help this question urgently?

Beatrice has some twenty-cent and fifty-cent coins. 2/5 of the total value of the twenty-cent coins is the same as 1/2 of the total value of the fifty-cent coins. If Beatrice spends 4 fifty-cent coins, the value of the twenty-cent coins and the value of the remaining fifty-cent coins will be the same. How many twenty-cent coins does Beatrice have? Ans 30.

Hi

Please confirm with your source.

Based on number in your question(2/5),the answer can’t be 30.

2/5 out of $6 (30 number of twenty-cent coins) is $2.40 (cannot be ½ of the total value of the fifty-cent coins).

If it is 2/3, then the answer of 30 is logical.

20 cents ----- $6 (30)

50 cents ------ $8

Best wishes

[Dazzle]

MathIzzzFun:

Dazzle:

http://i45.tinypic.com/2dvralg.jpg\"> Realised the earlier pic is way too small. see if this is big enough

http://i45.tinypic.com/29pra5u.png\">

cheers.

Thanks, Tianzhu and MathIzzzFun. Appreciate your help.

[fantasy1]

Hi



I need help for this qn :

Mr Tan is (8K + 10) yrs old.He has 2 children. Rosalind and David. Mr Tan is twice as old as Rosalind now. David is 2K years younger than Rosalynn. How old will Mr Tan be when David turns 18 yrs old ?

Thank You

[tianzhu]

Hi


You’ve also posted the same questions in the PSLE thread.

Your questions have been answered on page 826 of this thread.

http://www.kiasuparents.com/kiasu/forum/viewtopic.php?f=69&t=280&start=8250

Best wishes