英文數學ｇａ問題（急！！！）ｔｈｘ～～～

Alexander bought 30 boxes of cosmetics at $300. She found that 5 boxes were expired.If she sold out the remaining at $180 per box,find the profit per cent of this trade.
profit= [ (30-5)*180 - (300) ] / (300) * 100%
　　= 14%
A hawker sold 1500 pieces of key chains at a loss of20%. He lost a total of $6000.Find the cost price of each key chain.
Let $y be the cost price of each key chain.
1500y - 1500y*(1-20%) = 6000
1500y - 1500y*(0.8) = 6000
1500y (1-0.8) = 6000
1500y = 30000
y = 20
Therefore, the cost price of each key chain is $ 20.
Last year, the marked price and the selling price of a new digital camera were $7200 and $6120 respectively. But this year the marked price and the selling price are adjusted to $6400 and $5600 respectively. In comparing the discount per cent of these two years, which one is larger?
Last year discount cost = 6120 / 7200 = 0.85 = 85%
This year discount cost = 5600 / 6400 = 0.875 = 87.5%
Therefore, this year's discount is larger.
A computer game software marked at $280 is sold at 20% off. If the discount is 35% of its cost price, find the profit per cent.
Let the cost price be $y.
y*35% = 280*(1-20%)
y = 640
The cost price is $640.
profit = 280*(1-20%) / 640 = 0.35 = 35%
Alex deposits a sum of money in a bank at 5.5% p.a.Find the sum of money that Mr Mok deposits if he can get $2310 as simple interest after 3 years.
Let the sum of money be $y.
y*5.5%*3 = 2310
y = 1400
Therefore, the sum of money that he deposited is $1400.

2009-07-23 22:48:24 補充：
Kathy borrows $36000 from a finance company at 3.75% p.a.simple interest.If she has to return a total amount of $45450,how long does she borrow?

Let the time she borrows be y.
36000*3.75%*y = 45450-36000
y=0.7
Therefore, she borrows 0.7 year.

2009-07-23 22:49:12 補充：
A cat and a dog are not satisfied with their weights. If the weight of the cat is increased by 20% while that of the dog is decreased by 10%, their weights will be the same. What percentage of dog's original weight is cat original weight?

2009-07-23 22:49:16 補充：
Let the dog's weight be y and the cat's weight be x.
y(1-10%) = x(1+20%)
0.9y = 1.2x
x:y = 0.75 = 75%