For n months total plan A cost for both the lines:

\( 0+n( 40+15 ) ... \) installation charges are 0 and monthly rent are 40 and 15 respectively.

For n months total plan B cost for both the lines:

\(50+30 + n(35+10 )...\) Note: Installation charges are \(50 \) and \(30 \) respectively and are charged only once well as rent, monthly.

Also given total cost for n months for both the lines are equal for both the plans:

Hence :Both lines, total plan A cost for n months = Both lines, total plan B cost for n months

-> \(0+n( 40+15 ) = 50+30 + n(35+10 )\)

->\(55n =80+45n\)

->\(10n=80\)

->\(n=8\)

Ans C