How many litres of a 86 % concentric acid needs to be mixed with 78 %…
2025
How many litres of a 86 % concentric acid needs to be mixed with 78 % concentric acid so as to get 40 litres of 80 % concentric acid ?
- A.
12
- B.
13
- C.
11
- D.
10
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Show answer & explanation
Correct answer: D
Key idea: use a weighted average (or set up an equation) to find how much of the 86% solution is needed.
Let x be the litres of 86% acid. Then 40 - x litres are 78% acid.
Set up the equation for total acid: 86x + 78(40 - x) = 80 × 40.
Simplify: 86x + 3120 - 78x = 3200 → 8x = 80 → x = 10.
Therefore 10 litres of the 86% solution are required.
Alternative (alligation) quick check:
Difference between 86% and 80% is 6 (these are parts of the 78% portion).
Difference between 80% and 78% is 2 (these are parts of the 86% portion).
Ratio of 86% to 78% = 2:6 = 1:3, so 1/(1+3) × 40 = 10 litres of 86%.
Answer: 10 litres