**Solution:**

Given,

Ramkali saved Rs.5 in the first week and then started saving each week by Rs.1.75.

Hence,

First term, a = 5

and common difference, d = 1.75

Also given,

a_{n } = 20.75

Find, n = ?

As we know, by the nth-term formula,

a_{n} = a + (n − 1) d

Therefore,

20.75 = 5 + (n – 1) × 1.75

15.75 = (n – 1) × 1.75

(n – 1) = 15.75/1.75 = 1575/175

= 63/7 = 9

n – 1 = 9

n = 10

Therefore, n is 10.