Sunita clears a debt of Rs 6300 in 12 monthly instalments that form an AP.
Her first instalment is Rs 250. By how much does each instalment rise?

6300 = $\frac{12}{2}$$[2(250)+11d]=6[500+11d]$, so $1050=500+11d$, giving $11d=550$ and $d$ = Rs 50. Rs 45.83 comes from using 12$d$ instead of 11$d$ — twelve instalments have only eleven increases between them. Rs 525 is the average instalment $\frac{6300}{12}$, not the rise. Rs 55 fails the check, giving a total of Rs 6630.