Question 1844
For a 20% salt solution, one fifth (ie 20%) is salt. As we have 40g of the solution, then 8g (ie a fifth) must be salt.

we would work out the % by saying (8/40)*100 = 20%

However, we now will add some more salt. Call this unknown amount, x grams. We therefore increase the amount of salt by 8 and the total amount by 8 too. And the new salinity is 36%. So, we have {{{((8+x)/(40+x))*100 = 36}}}

Re-arranging this gives

{{{((8+x)/(40+x)) = (0.36)}}}

{{{(8+x) = (0.36)(40+x)}}}

{{{(8+x) = (14.4 + (0.36)(x))}}}

Rearranging this again gives

{{{(0.64)(x) = (6.4)}}}

hence x=10g of salt required.

We can see this in the formula...it is now (18/50)*100 which is 36%

cheers
Jon.