Question

Que no - 29 and 30..........

Answer 1

29)
let amount of water be x and amount of alcohol be y
x + y = 60l
y = 20l 
percentage of y in x+y = 20/60 *100 = 33.3%
We have only 33.3% of alcohol. to make it 50%, we need more 50% - 33.3% of alcohol. that is 16.7%
16.7%of x+y is 10l
Therefore, we need to add more 10l.

Answer 2

See, 60 + x is the final amount of the solution.
It already contains 20 litres of alcohol, so the conc. is 20/60. Now, to get alcohol by solution ratio to 50% or 0.5, use the formula -
$\frac{20 + x}{60 + x} = 0.5$
Therefore, on solving we get x = 10.
Now, for the next question, we should add x kgs of salt to increase the conc. The current conc. is 20/100 of 60 i.e. 12. To make it 40, we just use this formula -
$\frac{12 + x}{60 + x} = 0.4$
So we get x = 20kg.