Savings in the First week = Rs. 5.

Savings in the Second week 5 + 1.75 = Rs. 6.75

Savings in ‘n^{th} week is Rs. 20.75

∴ A.P. 5, 6, 75 , …………. , 20.75

a = 5, d = 6.75 – 5 = 1.75, a_{n} = 20.75.

n =?

a + (n – 1) d = a_{n}

5 + (n – 1) (1.75) = 20.75

5 + 1.75n – 1.75 = 20.75

1 .75n + 3.25 = 20.75

1.75n = 20.75 – 3.25

1.75n = 17.5

∴ n = \(\frac{17.5}{1.75}\)

∴ n = \(\frac{1750}{175}\)

∴ n = 10

∴ In the 10th week her savings becomes Rs. 20.75.