食塩の量を、数や文字式で表す練習 |
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(1)8%の食塩水300gの中には、食塩が何g入っているか。 |
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(式)300×0.08=24 (答え)24g |
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(2)3%の食塩水 ag
の中には、食塩が何g入っているか。 |
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(式) a ×0.03=0.03a (答え)0.03a g |
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問題1 |
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8%の食塩水300gと、3%の食塩水 agを混ぜて、7%の食塩水を作りたい。 |
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3%食塩水を何g混ぜればいいか。(a の値を求めなさい、ということ。) |
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(ア) |
下の表の空いているところに、数・式・文字式などを書き込みなさい。 |
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8%食塩水 |
3%食塩水 |
7%食塩水 |
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食塩水の量 |
300 g |
a g |
300+
a |
g |
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食塩の量 |
300×0.08 |
a
×0.03 |
(300+ a )×0.07 |
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g |
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g |
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g |
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食塩の量で |
… |
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式を作る |
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この食塩と |
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この食塩を足すと |
この食塩になる |
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24g |
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0.03a g |
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0.07(300+a) g |
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(イ) |
方程式を書きなさい。 |
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(式) 24+ 0.03a = 0.07(300+
a ) |
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(ウ) |
上に書いた方程式を解きなさい。 |
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24+ 0.03a =21+ 0.07a |
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0.03a −0.07a =21−24 |
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−0.04a =−3 |
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a =75 |
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(エ) |
この問題の答え |
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75g |
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ポイント |
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(1) |
食塩水全体の量のことを考えるのではなく、食塩の量に注目する! |
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(2) |
食塩の量が「どう変化したか」で式を作る! |
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※変化しない場合もある(水だけを混ぜた場合など) |
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計算上の注意 |
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(1) |
食塩の量の出し方… 食塩水の量×割合(0.08など)=食塩の量 |
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(2) |
8%は、0.08です。小数がやりやすいですが、分数 |
8 |
でもかまいません。 |
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