Probability help! Urgent!!!

First, let a be the number of bulbs produced by production line A.
Let b be the number of bulbs produced by production line B
Let c be the number of bulbs produced by production line C
(a)
As the production line A produced x bulbs
So, (x)/(x+b+c)=5/12...(1)
For production line B, (b)/(x+b+c)=2/5...(2)
From(1),
(x)/(x+b+c)=5/12
12x=5x+5b+5c
-5b=-7x+5c...(3)
From(2),
(b)/(x+b+c)=2/5
5b=2x+2b+2c
3b=2x+2c...(4)
(3)*2,
-10b=-14x+10c...(5)
(4)*5
15b=10x+10c...(6)
(5)-(6),
-25b=-24x
b=(24x)/25
So, the number of bulbs produced by production line B is (24x)/25.

(b)
From(a), (1)
(x)/(x+b+c)=5/12
From(a), b=(24x)/25
And from the question, c=440
So,
(x)/(x+((24x)/25)+440)=5/12
12x=5x+(24/5)x+2200
(11/5)x=2200
11x=11000
x=1000
So, the number of bulbs produced by production line A is 1000 bulbs.

(a)
Let n be the no. of bulbs produced by line B.

(No. of bulbs produced by line A) : (No. of bulbs produced by line B)
x : n = (5/12) : (2/5)
(5/12)n = (2/5)x
n = (2/5)(12/5)x
n = 24x/25

2015-01-04 21:32:53 補充：
(b)
Let m be the bulbs produced by line A.

(No. of bulbs produced by line A) : (No. of bulbs produced by line C)
m : 440 = (5/12) : [1 - (5/12) - (2/5)]
m : 440 = (25/60) : (11/60)
m : 440 = 25 : 11
11m = 11000
m = 1000

No. of bulbs produced by line A = 1000