1.**Chlorine(Cl₂)** and **Hydrogen(H₂)** are the gaseous products obtained on electrolysis of Brine.

2. The correct answer is **18%**.

Let the initial amount of the radioactive isotope be "a".

As its half-life is 10 days,

after first 10 days, the amount becomes reduced to half = a/2 = 50%

after next 10 days (i.e. after first 20 days), the residual amount becomes halved = (a/2)/2 = a/4 = 25%

after another next 10 days (i.e.after first 30 days), the residual amount becomes halved again = (a/4)/2 = a/8 = 12.5%

As 25 days end between the first 20 days and first 30 days, even the percentage of sample remaining after 25 days will be definitely, in between the respective values, i.e. in between 25% and 12.5%.

The only value which is neither 25% nor 12.5% but in between 25% and 12.5% is 18%.

**Hence, the percentage of the sample remaining after 25 days is 18%.**