ｌａｓｔ　ｍａｔｈｓ題唔該

50cents的金幣=N枚
$2金幣=(300 - N)枚

$0.5N + $2(300 - N) = $300
0.5N + 600 - 2N = 300
600 - 1.5N = 300
1.5N = 300
N = 200

$2金幣=300 - (200)
$2金幣=100枚

所以答案係 100枚。

Let a be the no of 50 c (i.e.$0.5) and b be the no of $2 coins :

0.5a+2b=300.......................(1)
a+b=300..............................(2)

(1) x 2 : 2(0.5a+2b)=300x2
a+4b=600................(3)

(3)–(2) : (a+4b)-(a+b) = 600-300
a+4b-a-b = 300
3b = 300
b=100

∴There are 100 $2 coins

設五毫(50c)為x, $2為y
得出兩條方程式.....
300 = x + y---------(1)
300 = 0.5x + 2y---------(2)

由(1)
y = 300 - x--------(3)

代(3)入(2)
300 = 0.5x + 2(300 - x)
x = 200
所以y = 100
$2金幣有100個

0.5N + 2 ( 300 - N ) = 300

0.5N + 600 - 2N = 300

-1.5N = -300

N = 200

Checking :

0.5( 200 ) + 2 ( 300 - 200 ) = 300

100 + 200 = 300

300 = 300


200枚是50c/金幣 , ( 300 - N ) = 100枚是 $2金幣.

(0.5 x N ) + ( 2 x (300 - N)) = 300
0.5N + (600 - 2N) = 300
0.5N + 600 - 2N = 300
-1.5N + 600 = 300
-1.5N = 300 - 600
-1.5N = -300
N = 200

0.5N+2(300-N)=300
1.5N=300
N=200
所以共有300-200=100枚$2金幣