Ramkali saved rupay 5 in the first week of a year and then increased her weekly savings by rupay 1.75. If in the nth week, her weekly savings become rupay 20.75, find n.

Savings for the first week = a = Rs. 5 increment in saving = d = Rs. 1.75
Let, her saving become Rs. 20.75 after n weeks.
Therefore, a_n = 20.75
⇒ a + (n -1)d = 20.75
⇒ 5 + (n -1)(1.75) = 20.75
⇒ (n -1)(1.75) = 15.75
⇒ n – 1 = 15.75/1.75 = 9 ⇒ n = 10
Hence, her saving become Rs 20.75, after 10 weeks.