Solution:
[tex]\text{mass percent} = \frac{\text{mass}_{\text{NaOH}}}{\text{mass}_{solution}} × 100[/tex]
[tex]\text{mass}_{\text{NaOH}} = \frac{\text{mass percent}}{100} × \text{mass}_{solution}[/tex]
[tex]\text{mass}_{\text{NaOH}} = \frac{40}{100} × \text{800 g}[/tex]
[tex]\boxed{\text{mass}_{\text{NaOH}} = \text{320 g}}[/tex]
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