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

Ramkali’s savings in the consecutive weeks are:

 ₹5, ₹(5 + 1.75), ₹(5 + 2×1.75), ₹(5 + 3×1.75) ... and so on.

We have,

a = 5, d = 1.75, a_n = 20.75

n^th weeks the savings will be, ₹[5 + (n - 1)×1.75] = ₹20.75

20.75 = 5 + (n - 1)1.75

15.75 = (n - 1)1.75

(n - 1) = 15.75/1.75

n - 1 = 1575/175

n - 1 = 9

n = 10

In the 10^th week, Ramkali will have saved  ₹20.75.