A tank holds 60 litres of a 40% acid solution. 15 litres are removed and replaced with 15 litres of an 80% acid solution. What is the strength of the solution now?

30%Correct answerYour answer48%Correct answerYour answer50%Correct answerYour answer60%Correct answerYour answer

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CorrectNot quiteAnswer C

Correct answer: C

Acid at first = $0.40 \times 60 = 24$ litres.
Removed: $0.40 \times 15 = 6$ litres of acid; added: $0.80 \times 15 = 12$ litres.
Acid now = $24 - 6 + 12 = 30$ litres in 60 litres, i.e. 50%.

Worth noting

48% adds the 80% solution without removing the 15 litres first. 60% is the plain average of 40% and 80%, and 30% treats the added liquid as water.

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