The money which Kanika puts in her piggy bank forms an AP with first term (a) as 1 and connom difference (d) as 1.
The month of January has 31 days.
So, the money which Kanika puts in her bank is the sum of 31 terms of the AP.
It is given that she spends Rs 204 from her pocket money and she has still Rs 100 with her.
Therefore, Kanika's pocket money = Rs 496+Rs 100+Rs 204 = Rs 800
We hope that clarifies your query.