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

Answer 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.