Maths Question 急

1 Let say now she is x years old
Then
x>2(x-7)
14>x
So her maximum age is 13
A
2
1-3x<8
-7<3x
3x>-7
x>-7/3
The no. of negative integer solution(s) is -2,-1
The answer is B
3
Let say the speed of Mandy is x m/min
Then since the total distance from home to school = 80*15=1200 m
Mandy use less than 12 mins to finish this distance
So the minimum walking speed of Mandy
=1200/12
=100 m/min

1.Let her maximum age now be x & her age 7 years ago be y.
x-7=y.......(1)
{
x=2y.......(2)

Put (1) into (2), Put x=14 into (1),
x=2(x-7) (14)-7=y
x=2x-14 y=7
x-2x=-14
-x=-14
x=14
Therefore may's maxinmum age now is 14& the answr is B.

2. Sorry I don't know too.But I think it maybe answer D.

3.the distance to walk to school
=15x80
=1200m

Let the minimum walking speed of Mandy be y.
1200/y=15+3
1200/y=18
y=1200/18
y=66/2/3
Therefore the minimum walking speed of Mandy is 66/2/3 m/min

2009-01-04 17:33:01 補充：
sorry question no.3 is wrong!!
this is is the right one!!

the distance to walk to school
=15x80
=1200m

Let the minimum walking speed of Mandy be y.
1200/y=15-3
1200/y=12
y=1200/12
y=100
Therefore the minimum walking speed of Mandy is 100 m/min.

1.A
let the ages of may is X now.
x>2(x-7)

2.D
1-3x<8
x>-7/3
-3 , -4 , -5 ..........

3.let the walking speed of Mandy be x.
the distance of the whole route
= 80* 15
=1200m

the used time of Mandy
=15-3
=12mins

the minimum walking speed of Mandy
=1200/12
=100m/mins
佢minimum既意思其實係指佢最小要行幾快先可以同May一齊到學校.