Let her pocket money be Rs x.

Now, she takes Rs 1 on day 1, Rs 2 on day 2, Rs 3 on day 3 and so on till the end of the month, from this money.

This makes a list

1, 2, 3, - - - , 31 [ as Jan has 31 days]

Clearly this is an AP with first term, a = 1 and d = 1 and no of terms = 31

So amount of money she puts in piggy bank in a month

Sum of n terms of this AP,

= 31(16)

= 496

So, Kanika puts Rs 496 till the end of the month from her pocket money

Also, she spent 204 of her pocket money and found that at the end of the month, she still has Rs 100 with her.

Now, according to the condition,

x - 496 - 204 = 100

x - 700 = 100

x = 800

So she gets 800 Rs as her pocket money.