勁易數學題,但我唔識

x: 被減數
y: 減數
z: 差

x-y = z ...1
x + y + z = 200 ...2
y = z-30 ...3

Sub 3 into 1
x-(z-30) = z
x-z+30 = z
x=2z-30 ...4

Sub 3&4 into 2
x+y+z = 200
(2z-30)+(z-30)+z=200
4z-60=200
4z=260
z=65
ie.差=65

actually 樓上個位ge方法係正確
不過係計錯
35+30=65
not 75

設減數係x 被減數係y 差係z
y - x = z-------(1)
x + y + z = 200--------(2)
x + 30 = z -------(3)

(3)代入(1),
y - x = x + 30
y = 2x + 30---------(4)

(3)同(4)代入(2),
x + (2x + 30) + (x + 30) = 200
4x + 60 = 200
4x = 140
x = 35

將(x=35)代入(3)
(35) + 30 = z
z = 75

所以差係75

Let x be 減數, y be 被減數 & z be 差

x - y = z ... (1) (減數 - 被減數 = 差)
x + y + z = 200 ... (2) (被減數,減數和差的和共200)
x + 30 = z ... (3) (減數比差少30)

x - y = x 30 ... (1) & (3)
-y = 30
y = -30

x + (-30) + z = 200 ... (2)
x + (- 30) + z = 200
x + z = 230
z = 230 - x

x - y = 230 - x ... (1)
x - (-30) = 230 - x
x + 30 = 230 - x
2x = 200
x = 100

x - y = z ... (1)
100 - (-30) = z
100 + 30 = z
z = 130

Therefore, 差是130

if z=差 , x= 減數

x+z=200
z-x=30
即係z=30+x

x+z=200
x+30+x=200
30+2x=200
2x=170
x=85

z-x=30
z-85=30
z=115

所以差是115。