我想問幾條數學問題!!10點!!!

1. Let x be the number of boys and y be the number of girls.
x/y = 3/5----1
y = x+8----2x = 12 and y = 20, the total number is 32
Using substitution method, you should find the value of x and y.
Hence, you can find the total number of boy scouts and girl guides.
Ans: x = 12, y = 20 and the total number = 32
2. Let x kg be the weight of the cat and y kg be the weight of the dog.
x(1+20%) = y(1-10%)
Ans: y/x = 4:3
3. I think the answer should be A. (not sure)
Here is my explanation:
Let 1 = 4/4
The second number = 5/4 = (4+1)/4
The third number = 7/4 = (4+3)/4 = (5+1*2)/4
From above, we can see that the numerator of the third number
= 2+numerator of the second number*2
Using this property, we can find out the answer easily.
4. Let 1pencil, 1pen, 1ruler and 1rubber be 1 set of stationary.
To find the answer, we have to find
1) the total cost of 1 set of stationary
2) the greatest prime number which is smaller than 120.
By trials , the cost of 1set of stationary is $24 and the required prime number is 113.
If we buy 1set of stationary, the remaining money we have = 113 - 24 = $89
If we buy 2set of stationary, the remaining money we have = 113 - 24*2 = $65
If we buy 3set of stationary, the remaining money we have = 113 - 24*3 = $41
If we buy 4set of stationary, the remaining money we have = 113 - 24*4 = $17
Since we don't have enough money onwards, the list can be stopped.
From above, only 65 is divisible by 5. Therefore, we should consider 2sets of stationary.
65/5 = 13
Therefore, he can buy 13+2=15 pencils at most.
5. To find x, you have to know the following formula:
I = PRT, where
I = interest
P = principal(money you deposit)
R = interest rate per annum
T = time(year)
The equation should be 2310 = x*5/1/2%*3
Answer: x = $14000