maths 1

this is Q of 2 equations of 2 unknowns
Method 1 :
By elimination :x+y=20------(1)
2x+y=50 -----(2)
You can observe that the y can be cancelled by (2) minus (1), it becomes a linear equation in 1 unknown (x only)
(2)--(1): 2x-x=50--20
x=30
Put x=30 to (1)
Reason : (1) is more simple than (2)

y=20-x=20-30=-10

Method 2 :By substitution :
For both 1 and 2 , express y in terms of x :
y=20-x
y=50-2x
Equate them together : 20-x=50-2x
x=30 and y=20-30 or 50-2(30)=-10

x+y=20------------(1)
2x+y=50----------(2)
(2)-(1),
2x+y-(x+y)=50-20,
2x+y-x-y=30,
2x-x+y-y=30,
x=30
將 x 代入 (1),
30+y=20,
y=-10
Answer, x=30, y=-10

x+y=20
x=20-y

2x+y=50
2(20-y)+y=50
40-y=50
y= -10

x+y=20
x-10=20
x=30

x+y=20
x=20-y

2x+y=50
2(20-y)+y=50
40-y=50
y= -10

x+y=20
x-10=20
x=30

x+y=20............1
2x+y=50..........2

1-2 x+Y-2X-Y=20-50
-X=-30
X=30

30+Y=20
Y=-10

x+y=20 -(1)
2x+y=50 -(2)

(2) - (1): (2x-x) + (y-y) = (50-20)
x =30

in (1) 30 + y =20
so y = -10

Checked the results in (2)
2 * 30 + (-10) = 50

Answers:
x = 30
y = -10