help me 有數學功課唔識

Let the no. of 50 cent coins be a
Let the no. of $2 coins be b
(0.5)a + (2)b = 42 ……(i)
a + b = 30
b = 30 - a
Put (b = 30 - a) into (i), we get
(0.5)a + (2)(30 - a) = 42
0.5a + 60 - 2a = 42
60 - 42 = 2a - 0.5a
18 = 1.5a
18 / 1.5 = a
12 = a
a = 12
Thus, b = 30 - a = 30 - 12 = 8

Ans. There are 12 (50 cents) coins and 8 ($2) coins.

2006-11-04 15:45:32 補充：
Correction:Thus, b = 30 - a = 30 - 12 = 18Ans. There are 12 (50 cents) coins and 18 ($2) coins.

Let x be the number of $50-cent coins ;
Let y be the number of $2-coins

0.5x + 2y = 42 ----- (1)
x + y = 30 ------------(2)

From (2) : x = 30 - y ------(3)

Sub (3) into (1),
0.5 (30 - y) + 2y = 42
15 - 0.5y + 2y = 42
1.5y = 27
y = 18 ------(4)

Sub (4) into (2),
x + 18 = 30
x = 12

Hence there are 12 $50-cent coins & 18 $2-coins.

2006-11-04 15:54:06 補充：
唔清楚你幾年班, 老師容不容許你跳steps,所以我盡量將所有steps列出....我個時老師要求將每個計算中產生的assumption都用數字標示....唔知你老師要唔要

2006-11-04 15:55:20 補充：
不過實在年代太久遠....可能都漏咁幾個步驟....

Let X be the number of $50-cent coins
and (30-X)be the number of $2-coins

0.5X + 2(30-X) =42
0.5X +60-2X =42
1.5X =18
X=12
the number of $50-cent coins and $2-coins are 12 and 18.

2006-11-04 15:47:11 補充：
上面果個人　搞到好複雜我果個似ｆ．１～２　諗　我估你ｆ．１～３到　係未？？不過你鍾意邊個ａｎｓ　都冇所謂

我唔會幫你做,但我幫你解...
有30枚$50分硬幣和$2硬幣係個錢箱，硬幣的總價值是$42。 平均每個衡量單位幾多個硬幣在錢箱？

2006-11-04 15:43:12 補充：
因為我都唔你好識做(只係其中一個原因)

2006-11-04 15:44:01 補充：
打錯,,,係"因為我都唔係好識做"