點計? (比條式)
1.Ben has 20 coins which consist only $2 and $5.If the total amount of these coins is not less than $85,how many $2-coins can he have at most?
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Mathematics........
2010-06-01 8:59 pm
回答 (2)
2010-06-01 9:55 pm
✔ 最佳答案
Let x be the number of $2 coin and (20-x) be the number of $5 coinIf the total amount of these coins is not less than $85
2*x+5*(20-x)>=85
2x+100-5x>=85
3x<=15
x<=5
Hence, he can have five $2-coins at most.
2010-06-01 13:55:33 補充：
>= means "greater than or equal to"
<= means "smaller than or equal to"
2010-06-01 9:55 pm
Let x be the number of $2 coin and y be the number of $5 coin.
Then, Ben has 20 coins : x + y = 20 ---------- (1)
total amount of these coins is not less than $85 :
2x + 5y >= 85 ---------- (2)
From (1) : y = 20 - x ---------- (3)
Sub. (3) into (2) : 2x + 5(20 - x) >= 85
2x + 100 - 5x >= 85
100 - 3x >= 85
3x <= 15
x <= 5
So, Ben can have 5 $2 coins at most.
Then, Ben has 20 coins : x + y = 20 ---------- (1)
total amount of these coins is not less than $85 :
2x + 5y >= 85 ---------- (2)
From (1) : y = 20 - x ---------- (3)
Sub. (3) into (2) : 2x + 5(20 - x) >= 85
2x + 100 - 5x >= 85
100 - 3x >= 85
3x <= 15
x <= 5
So, Ben can have 5 $2 coins at most.
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