For a) take note that that the amount of water doesn't change, since the questions only indicates that we need to add in salt.

Hence, we can use this to help us solve the question

I think the previous user who answered used a different method to solve part a. I went by fixing the proportion for salt then finding out how much salt needed for the remaining proportion of water to be 40%.

notes:
part a: adding mass of salt affects total mass so we cannot just take 6*0.6 to find how much more salt to add.

qs2: take note of what qs says, as the interest rate is per month, so n should be 1 and t=10 months using compound interest formula. (Also, it starts at the end of February, so February should not be added into t, that's why t=10)

For the compound interest qn,use the compound int formula (should be in formula sheets during exams).

First find out the yearly interest rate which is 2% x12. Convert that to decimal, as per the formula.

Then calculate how many months the loan is for,which is 10 months. Typically t should be in years and n is the num of times interest is compounded per year

Since interest is compounded monthly. It would be compounded 12 times per year. N =12.
10 months is 10/12 = 5/6 of a year. Which is t.

Then just compute. :)