中二數學問題三條!20點!要詳細步驟!

14.
a)
Let b be the number of boys
b/b+22=3/5
b=33
Total number of participants=33+33+22=88
b)
NO
new ratio=(33-4):(33+22-4)
=29:51

15
假設今年爸爸x歳,言雅y歳
x=7y---(1)
10+x=3(10+y)
x=20+3y---(2)
Put (1) into (2)
7y=20+3y
y=5
今年言雅5歳,爸爸35歳
假設r年後爸爸的年紀是言雅的2倍
35+r=2(5+r)
r=25
所以25年後爸爸的年紀是言雅的2倍

19
a)
L=ab+(1+20%)ab
=2.2ab
b)
L=2.2ab
b=L/2.2a
c)
No
if they meet after 2.5 hour,
distance(L km) = (2.2a)2.5 = 5.5a km
so,lorry I complete the journey in 5.5 h

我已解答，但被指違規：

請看我的答案：

http://postimg.org/image/k39nyc6tz/

2014-02-05 14:34:15 補充：
Man fu，我己經 send 了 email 給你，請看看。

2014-02-05 14:35:21 補充：
14)
(a) Let the number of boys be 3k and number of girls be 5k where k is a constant.
3k = 5k - 22
2k = 22
k = 11
The number of boys is 3k = 33
The number of girls is 5k = 55
The total number of participants in the singing contest is 33 + 55 = 88.

2014-02-05 14:35:30 補充：
(b) If 4 boys and 4 girls quit the contest, there are
33 - 4 = 29 boys and
55 - 4 = 51 girls.
The ratio is 29 : 51 which is different from 3 : 5.
Therefore, the ratio of the numbers of boys and girls does NOT remain unchanged.

2014-02-05 14:35:53 補充：
15) 設 x 為爸爸現在的年紀, y 為言雅現在的年紀。
{x = 7y
{x + 10 = 3(y + 10)

7y + 10 = 3(y + 10)
7y + 10 = 3y + 30
4y = 20
y = 5
x = 7y = 35

爸爸現在的年紀是 35 歲。
言雅現在的年紀是 5 歲。

設現在起 n 年後, 爸爸的年紀將是言雅的2倍。
35 + n = 2(5 + n)
35 + n = 10 + 2n
n = 25

所以, 現在起 25 年後, 爸爸的年紀將是言雅的2倍。

2014-02-05 14:36:21 補充：
19)

(a) The speed of Lorry 2 is 1.2a km/h.
In b hours, both lorries travel in total L km.
Therefore, a × b + 1.2a × b = L
(a + 1.2a) × b = L
(2.2a) × b = L
L = 2.2ab

2014-02-05 14:36:35 補充：
(b) L = 2.2ab
b = L/(2.2a)

2014-02-05 14:37:05 補充：
(c) Given that b = 2.5.
2.5 = L/(2.2a)
L = 2.5 × 2.2a = 5.5a
The time Lorry I takes to complete the journey is
Distance / Speed
= L / a
= 5.5a / a
= 5.5 hours
Therefore, Lorry I cannot complete the journey in 5 hours.

2014-02-05 14:37:41 補充：
奇怪！　完全不知道哪裏違規？！？！