Question

how much alcohal must be added to 500ml of a 16percent solution to make its strength 30 percent

Answer 1

Amount of alcohol in solution = 500 × 16/100 = 80 ml.
Let after adding x ml alcohol, strength becomes 30%.
Then,
$\frac{80 + x}{500 + x} \times 100 = 30 \\ 8000 + 100x = 15000 + 30x \\ 70x = 7000 \\ x = 100 \: \: ml$