Salt, pepper and everything nice

Let the amount of a tablespoon pepper and a spoon salt be x g.

Using the formula of percentage by mass= $\frac{Mass of solute}{Mass of solution}\times100\%=\frac{Mass of solute}{Mass of solute+Mass of solvent}\times100\%$ ,

We have

$\frac{x}{x+400}\times100\%=50\%$

$\frac{x}{x+400}=\frac{1}{2}$

$2x=x+400$

$x=400$

Hence, the mass of solute (a tablespoon pepper and a spoon salt)= $400g$

The concentration later= $\frac{400\times(3+1)}{400\times(3+1)+400}\times100\%=\frac{1600}{2000}\times100\%=80\%$

So, the change in concentration (percentage by mass)= $80\%-50\%=30\%$

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Note: Unless it is stated in the question, all the solute added cannot be extracted.

So, the concentration later= $\frac{400\times(3+1)}{400\times(3+1)+400}\times100\%=\frac{1600}{2000}\times100\%=80\%,$

instead of the concentration later= $\frac{400\times3}{400\times3+400}\times100\%=\frac{1200}{1600}\times100\%=75\%.$