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Answer

Hi Farhaini,

The first step would be to identify what chapter this question belongs to. The question asks **how much of each solution**, so from there we can conclude it’s a question with multiple unknowns, thus this has something to do with simultaneous equations.

The next helpful step is to simplify the given information into a table form (or however you like that makes it easy to you)

We know that the 3 volumes used must add up to 500L. I think that the question should’ve phrased it in a better way to avoid confusion: the volume of the 10% acid **used** (let’s call it x) is twice of that of the volume of 40% acid used. (Let’s call it y)

Since we know that x=2y, we can directly substitute it so that it is an equation with 2 unknowns.

The other equation is a bit hard to explain- since I took the old SPM syllabus, I could only think of an explanation in terms of chemistry, maybe under the new syllabus there is another explanation for it though

It’s like adding a moles from solution 1, b moles from solution 2, and c moles from solution 3, and the total number of moles in the solution is a+b+c.

From chemistry we know that

[ moles=MV= molarity × volume ]

I would convert the 10%, 40%, and 60% into a decimal form: 0.10, 0.40 and 0.60 respectively.

So the second equation would be in the form of:

M_1V_1 + M_2V_2 + M_3V_3=M_{total}V_{total}

We need not worry about the units here as it’s the same on both LHS and RHS.

And the last step is quite standard:

So the final answer:

Fifi should use 125L of 10% acid solution,

62.5L of 40% acid solution and

312.5L of 60% acid solution.

Personally find this question kind of poorly phrased and unfair haha

Hope this helps!