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Several primary school math problems
One,

1. mass of salt = (80 * 5%+20 * 8%) * (80+20-10)/(80+20) = 5.04g.

2. Salt water quality = 80+20-10+10 =100g.

3. Salt content% = 5.04/100 *100% = 5.04%.

Second,

1. Let the radius of the outer circle be r and the radius of the inner circle be r, then R:r=3:2, and if it is known that R=6cm, then r=4cm.

2. Semi-cylindrical area = 0.5 * 3.14 * 6 * 6 = 56.52cm2.

3. Office area = 0.5 * 3.14 * 4 * 4 = 25.12cm2.

4. Semicircular area = 56.52-25.12 = 31.4cm2.

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