英文數學(知答案,唔識列式)

I spent 205 of my money and then 25% of the remainder. If I have $30 left, how much did I have at first?

答:(x-205)X(1-25%)=30
Or
30除75%+205=245

以上既野我咩都唔知

仲有我個老師話冇可能$50

應該係先用20%, 再用餘下的25%後, 剩下$30.
I spent 20% of my money and then 25% of the remainder. If I have $30 left, how much did I have at first?

Let there are at first $x.

The remainder of the money after 20% is spent
= x * (1 - 20%)
= x * 0.8
= 0.8x

$30 is left after 25% of the remainder is spent
=> 0.8x * (1 - 25%) = 30
=> 0.8x * 0.75 = 30
=> 0.6x = 30
=> x = 30/0.6 = 50

There are at first $50.


Remarks: Complex equation
x * (1 - 20%) (1 - 25%) = 30
=> x * 0.8 * 0.75 = 30
=> 0.6x = 30
=> x = 30/0.6 = 50

答案點會是 $50, 你的問題是 用咗 205, 然後再用餘下的25%, 就餘 $30. 那當 X 是當初的金額, 條式是 (X-205) (1-25%) =30, 答案應是 $245.

你個問題係咪唔岩,
你用左$205,仲淨番$30,點會一開始得$50-_-
唔明個問題,有冇清楚d

let x be the money you have at first
(x-205)x(1-25%)=30
x-205=40
x=245