有幾條數學問題20點!!

1.
4 + 5a = -16
5a = -20
a = -4

2.
3 - 4x = 7
-4x = 4
x = -1

3.
2z / 15 = 1 / 5
15(2z / 15) = 15(1 / 5)
2z = 3
z = 3/2

4.
3y - 5 / -2 = 7
3y - 5 = -14
3y = -9
y = -3

5.
12 - 2y = y
-2y = y - 12
-3y = -12
y = 4

6.
t / 9 = 7- t / 12
36(t / 9) = 36(7- t / 12)
4t = 3(7 - t)
4t = 21 - 3t
7t = 21
t = 3

7.
1+ 7x / 4 = 9x / 5
20(1+ 7x / 4) = 20(9x / 5)
5(1 + 7x) = 4(9x)
5 + 35x = 36x
35x = 36x - 5
-x = -5
x = 5

8.
2k / 5 - 4k / 9 = -2
45(2k / 5) - 45(4k / 9) = 45(-2)
9(2k) - 5(4k) = -90
18k - 20k = -90
-2k = -90
k = 45

9.
-3y / 8 + 2y / 3 = 7
24(-3y / 8) + 24(2y / 3) = 24(7)
3(-3y) + 8(2y) = 168
-9y + 16y = 168
7y = 168
y = 24