數學問題 中一 help!!

1) York has $72.Sally has $12.In order to let Sally have 3 time the amount of
York, how much should York give to Sally?

Let $ x be the amount of money that York give to Sally.
3 (72 - x) = 12 + x
216 - 3 x = 12 + x
- 3 x - x = 12 - 216
- 4 x = - 204
x = 51
∴the amount of money that York give to Sally is $51.

2)Carl has $9 more than David does. Hilbert has 3 of the amount of Carl. If they have a amount of $61, how much does each of them have?

Let $ x be the amount of money that Carl has. Then x +9 is the amount of money that David has, 3 x is the amount of money that Hilbert has.
x + x + 9 + 3 x = 61
5 x = 61-9
5 x = 52
x = 10.4
The amount of money that David has= 10.4 + 9
= ($) 19.4
The amount of money that Hilbert has= 3×10.4
= ($) 31.2
∴Carl has $10.4, David has $19.4 and Hilbert has $31.2 .

3)A submarine dives vertically from -34m to-84m and then rises vertically to-9m.Find the total distance travelled.
The total distance of a submarine travelled
= (- 34) + [(- 84) - (-34)] + [(- 84) - (-9)]
= (- 34) + [-84+34] + [-84+9]
= (- 34) + (- 50) + (- 75)
= - 34 - 50 + 75
= - 159(m)
∴the total distance of a submarine travelled is 159m.

I try as follows : -

1. F : 72 - x = (x+12)3
72 - x = 3x +36
72-36 = 4x
9 = x

2. F : x + (9+x) + 3(9+x) = 61
x + 9 + x + 27 + 3x =61
2x +3x +36 = 61
5x = 61 - 36
x = 5
David has $5, Carl has $14, Hibert has $42

3. F : -84m+(-84-9)m
=- 84m + (-93)m
= - 177m