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

Weekly savings of Ramkali is following :
5, 6.75, 8.50, 10.25............
Here, a = 5, d = 1.75, T_n = 20.75
We know that, T_n = a + (n - 1) d
 20.75 = 5 + (n - 1) (1.75)
 20.72= 5 + 1.75 n - 1.75
 20.75 = 1.7n + 3.25
  1.75n = 17.5

     

Hence,    n = 10.