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)