奧數題目唔識做..!!!!!!! 20X2 MARKS FAST!!

1. ABCDx4=DCBA
Since ABCDx4 is a 4 digit number, ABCD<3000, hence A=1 or 2. But DCBA is a multiple of 4, hence even. Therefore A=2.

ABCD>2000, DCBA=ABCDx4 > 8000. Hence D=8 or 9. Since the unit digit of ABCDx4 is 2, therefore D=8.

Now we have 2BC8x4=8CB2. note that when Bx4, there can be no 進位 (since 2x4 is already 8), hence B can only be 0, 1 or 2. Moreover, B is the unit ditit of Cx4+3, hence B is odd. Therefore B=1.

21C8x4=8C12, and the unit digit of Cx4+3 is 1. Hence the unit digit of Cx4 is 8. Therefore C can only be 2 or 7. By check these two cases we see that C=7. Hence
ABCD=2178.

2.
7 17 3
5 9 13
15 1 11

3. Let the age of the sister now be x. Then the age of the brother now is 2x.
When the age of the sister is 2x, the age of the brother will be 3x.
Hence 3x=15
x=5.

4. Let there be n children, and the price be p.
p=8n-3 (1)
p=7n+4 (2)
(1)-(2), 0=n-7. So n=7, and p=53.

13
分析得Ａ＝１ｏｒ２同Ａ係偶數
所以Ａ＝２
得Ｄ＝８ｏｒ９
９＊４＝３６　個位唔係２
所以Ｄ＝８
因為Ｃ＊４＋３係單數
Ｂ＝１，３，５，７，９
代入（１００＊Ｂ＋１０＊Ｃ）＊４＋３０＝１００＊Ｃ＋１０＊Ｂ
Ｃ的答案要整數（０～９）
得Ｂ＝１，Ｃ＝７
ＡＢＣＤ＝２１７８
14
０７　１７　０３
０５　０９　１３
１５　０１　１１
15
設 兄現在x歲,妹現在y歲
ｘ－ｙ＝ｙ
15－ｘ＝ｙ
ｘ＝10，ｙ＝5
妹現在５歲
16
設　有小孩Ｘ人，玩具為Ｙ元
８＊Ｘ＝Ｙ＋３
７＊Ｘ＝Ｙ－４
Ｘ＝７（人），Ｙ＝５３（元）
玩具值５３元

13.

ABCD=2178


14.

03 11 13
17 01 09
07 15 05


15.

妹妹現在8歲


16.

玩具值53元

你打錯勁多字lor...睇唔明= =

16.設共有n名小朋友 和 玩具值x元
8n=x+3...(1)
7n=x-4...(2)
(2)-(1)
n=7

8(7)=x+3
56=x+3
x=53

玩具值53元

13. ABCD x 4 = DCBA
ABCD=DCBA/4

14.17 19 21
23 25 27
29 31 33

15.

15 When兄和妹妹現在的歲數同樣時,妹妹剛出生; 如果妹妹的歲數和兄現在的歲數同樣時,兄將15歲。妹妹現在幾歲?


妹妹現在15歲。