We know that:
3x + 10y = 7
We also know that:
x - 10y = 29
Which means that:
x = 29 + 10y
If, x = 29 + 10y,
3(29 + 10y) + 10y = 7
3*29 + 30y + 10y = 7
3(20+9) + 40y = 7
60 + 27 + 40y = 7
87 + 40y = 7
40y = 7 - 87
40y= -80
y=(-80)/40
y=-2
Answer y=-2.
If y=-2,
x-10(-2)=29
x+20=29
x=29-20
x=9
Let's see if these results are true:
3(9)+10(-2)=7
27-20=7 (Correct)
-----------
9-10(-2)=29
9+20=29 (Correct)
Answer once again:
y=-2
