Numerical answer
A container holds 54 litres of a mixture of acid and water in the ratio 7:2. How many litres of water must be added so that the new ratio of acid to water becomes 7:5? (Answer in litres)
Show answer and solution
Answer: 18
Acid = 54 × 7/9 = 42 L, Water = 54 × 2/9 = 12 L (acid quantity stays fixed as only water is added). Let x litres of water be added: 42/(12+x) = 7/5 ⟹ 42×5 = 7×(12+x) ⟹ 210 = 84+7x ⟹ 7x = 126 ⟹ x = 18. Check: 42:30 = 7:5. ✓
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