一條很有趣的問題!

設袋中最少有x個$5硬幣,
$2硬幣有(x-3)
5x +2(x-3) greater than or equal to 100
7x greater than or equal to 106
x greater than or equal to 106/7
x greater than or equal to 16
所以袋中最少有16個$5元硬幣.

Let the number of $2and $5 are A and B
1.2A+5B>=100
2.B-3=A

Put 1 into 2:
2A+5(A+3)>=100
2A+5A+15>=100
7A>=85
A>=12

So,the amount of $2 is 13x2=26,and at least (100-26)/5=17 $5 in the bag

袋中載有$2和$5硬幣,$5硬幣比$2硬幣多3個。若硬幣的總值不少於$100,問袋中最少有多少個$5硬幣?(請列式)
Let the number of $2and $5 are A and B
1.2A+5B>100
2.B-3=A
Put 1 into 2:
2A+5(A+3)>100
2A+5A+15>100
7A>85
A>12
So,the amount of $2 is 13*2=26,and at least 17 $5 in the bag (100-26)/5

設有x 個$2 則有x+3個$5
2x+5(x+3)>100
7x>97
x>97/7
x(min)=14
所以最少有17個$5

設x為5元硬幣數目，y為2元硬幣數目。
5x+2y>100
y+3=x
5x+2x-6>100
7x>106
x>15又7分之1
所以袋中最少有16個5元硬幣。

袋中有 x 個 $5 硬幣
袋中有 x - 3 個 $2 硬幣
5x + 2(x - 3) >= 100
7x >= 106
x >= 15.14
袋中最少有 16 個 $5 硬幣

2008-03-17 20:57:44 補充：
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