MATH....HELP(方程)

No.1

The number of students who travel by MTR is 400

Let X be the total number of the students in the school.

We have (1- 1/5 - 2/5)X = 400
2/5 X = 400
X = 1000

Number of students who travel by MTR = (2/5)(1000) = 400

No.2

The total savings of the man is $8000.

Let Y be the total savings of the man, we have

(1- 3/8 - 1/4)Y = $3000
3/8 Y = $3000
Y = $8000

No. 4

Reena is now 33 years old.

Let Z be the age of the son of Reena. Then Reena now ages 3Z.
Ten years later, her son will age (Z+10) and Reena (3Z+10).

We then have, (Z+10)+ (3Z+10) = 64
4Z + 20 = 64
4Z = 44
Z =11
Therefore,the son of Reena now ages 11 and Reena 33.

Both no. 1 and 2 are the same kind. Here are the solution
No.1
let all students in school is x, then
x-(1/5)x-(2/5)x=400
x-(3/5)x=400
(5/5)x-(3/5)x=400
(5-3/5)x=400
(2/5)x=400
2x=2000
x=1000
From the question, the number of students who travel by MTR are 1000*2/5=400.
No.2
let all saving is x, then
x-(3/8)x-(1/4)x=3000
x-(3-2/8)x=3000
(8-1/8)x=3000
(7/8)x=3000
7x=24000
x=24000/7
Hence,the total savings of the man is nearly 3428.57
No.3
let Reena is x and her son is y, then
x=3y(Reena is three times as old as her son)(1)
10 years later
(x+10)+(3y+10)=64(the the sum of their ages will be 64 years)
x+3y=44(join with (1))
3y+3y=44
6y=44
y=22/3 (7.33)
x=3y=3*(22/3)=22
hence, the present age of Reena is 22.

1.400=2/5 of sch.=MTR
2.3000!?
let x be the present age of Reena,
x/3+x+20=64
x+3x+60=192
4x=132
x=33
Reena is 33 yr now