Q&A - PSLE Math

[Ender]

Clever. The brighter kids would have seen it.

I think a similar question appeared in GEP selection test.

Basically, if you fold the four corners of the bigger square towards the centre, you will get the smaller square.
Therefore, bigger square is 2x of smaller square, ie, 288 sqcm

=> (2r)² = 288
=> r² = 288/4

Therefore pi*r² = 72pi

No wonder I did the 'long winded' method. I won't qualify for GEP..

[lulu2014]

how many ways can $1.50 be exchanged with 10 cents and 20 cents coins?

[Nebbermind]

how many ways can $1.50 be exchanged with 10 cents and 20 cents coins?

8 gwah...

Starting with 15 nos. ten cents, you can swop two ten-cents for 20-cents seven times.

[lulu2014]

but 8 is the wrong ans..

[Nebbermind]

but 8 is the wrong ans..

And the correct answer?

[EC72]

The figure below shows 2 quarter circles and a rectangle. The radius of the big quarter circle is 8 cm. The radius of the small quarter circle is 4 cm. Find the difference in area between the two shaded parts of X and Y. (Take pi as 3.14)

I would solve the question as follows:

Observe that the area of the rectangle less that of Y is also part of the area of the big quarter.
Thus, the difference in area between the two shaded parts of X and Y -> area of big quarter - area of small quarter - area of rectangle = (3.14 x 8 x 8)/4 - (3.14 x 4 x 4)/4 - (8 x 4) = 5.68sq cm

Ans : 5.68 sq cm.

alfretztay

[EC72]

Alternatively,

(i) B + C = Area of Square - Area of Bigger Qtr
(ii) A + B = Area of Rect - Area of smaller Qtr

(ii) - (i) => A - C

Nebbermind! Easier to understand this method.

[lulu2014]

The teacher didnt give any answer to my niece. only ask her to do corrections

[Nebbermind]

but 8 is the wrong ans..

If the question means 10-cents AND 20-cents, then seven ways since All 10-cents cannot be counted.

[Laurel111]

Can someone help with part (b) of this question? Ans: (a)7.74cmsq (b)9.12cmsq. Thanks.