Question

Poonam saved $Rs5$ in the first week of the year and then increased her weekly savings by $Rs1.75$ each week. In which week will her weekly saving be $Rs20.75$?

Answer 1

b'

Saving made in first week$=5$

Saving made in second week$=5+1.75=6.75$

Saving made in third week$=6.76+1.75=8.50$

So, the series is $5,7.75,8.50,...$

Since difference is same, it is an A.P

Common difference$=d=1.5$

Given:In $n^{th}$ week, her savings become Rs.$20.75$

So, $a_n=20.75$

Also,$a=5,d=1.75$

Using $a_n=a+left(n-1right)d$

$Rightarrow20.75=5+left(n-1right)times1.75$

$Rightarrow15.75=left(n-1right)times1.75$

$Rightarrown-1=dfrac15.751.75=9$

$Rightarrown-1=9$

$thereforen=10$

Hence, in ${10}^{th}$ week, her weekly savings become Rs.$20.75$

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