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標題: 请教数学题 [打印本頁]

作者: jilin1960 時間: 13-3-10 17:00 標題: 请教数学题

there are some $1,50centand 20-cent coins .the number of $1coins is 2 less than twice that of 50 -cent coins and the number of 20-cent coins exceeds that of $1coins by 6.if the total value of these coins is $27.8,what is the number of $1coins ?

thanks

作者: cowmoon 時間: 13-3-10 17:30

回復 jilin1960 的帖子

Which grade's question?
For older kids you may use algebra.
For younger kids you may use guess and trial.

$27.8 => in order to make 80 cents, the no. of 20 cents should be 4 or 9 or 14 or 19 or 24 or 29 ...
Because $1 is 6 less than 20 cents and $1 is around double of 50cents, in order to make $27.8 let's guess there is around 20 $1 ...

Answer:
24 x 20cents
18 x $1
10 x 50 cents

作者: jilin1960 時間: 13-3-10 18:56

回復 cowmoon 的帖子

谢谢！请问用代数如何解呢？

作者: cowmoon 時間: 13-3-10 19:01

Let n be no. of 50cents

based on info given,
no. of $1 = 2n - 2
no. of 20cents = 2n - 2 + 6 = 2n+4

therefore, 27.8 = n X 0.5 + (2n - 2) x 1 + (2n + 4) x 0.2

作者: smallfoothk 時間: 13-3-11 11:12

是數學教科書的練習嗎?如是,想知出版社?一般小學生太難解答.

作者: JustAParent 時間: 13-3-11 13:31

題中其實只有一個變數(5毫), 這類題目高小學生可應付的.

考慮只有一個5毫, 總額是0.5*(1) + 1*(2*1-2) + 0.2*(2*1-2+6) = 0.5 + ０+ 1.2 = 1.7

考慮只有二個5毫, 總額是0.5*(2) + 1*(2*2-2) + 0.2*(2*2-2+6) = 1 + 2 + 1.6 = 4.6

即增加一個5毫, 總額增加 4.6 - 1.7 = 2.9

(27.8-4.6)/2.9 = 8

即5毫共2+8=10個, 其他1元, 2毫也可算出.

這是雞兔同籠類的問題.

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