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A 400 g solution is 25% salt. How much salt must be added to produce a solution that is 40% salt?

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  1. 400 g solution is 25% salt, i.e. 100 g salt in 300 g water.
    A solution of 40% (by weight) is composed of 40% salt, and 60% water.
    So for 300g water, we need 5*40=200 g of salt.
    We need to add 200-100=100 g of salt to the 400 g solution to make the solution 40%.
    Check: water 300g, salt 200g, total 500g
    Percentage (by weight) of salt = 200/500=40%.

    Note:
    This is very well as far as mathematics go. In practice, 100g of water can only dissolve 40g of salt, at 100°C. So 300g of water can only dissolve 120g of salt, with 80g undissolved.

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