Question 159971
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Non member: ($4)(# of Vd)
Member: ($21)+($2.50)(# of Vd)
Equating the 2:
{{{4*Vd=21+2.50*Vd}}} EQN 1
{{{4Vd-2.50Vd=21}}}
{{{1.5Vd=21}}} -----> {{{cross(1.5)Vd/cross(1.5)=cross(21)14/cross(1.5)}}}
{{{Vd=14}}} ---------------> no. of videos to rent should be MORE than 14 to justify membership.
Okay, plug in 14 to EQN 1:
{{{4*14=21+2.50*14}}}
{{{56=21+35}}}
{{{56=56}}}, see, at 14 rentals being a non member or a member will just be the same
Try 15 rentals:
{{{4*15=21+2.50*15}}}
{{{60=21+37.50}}}
{{{60=58.50}}}, see, the higher the rentals, you start to save  on being a member.
Try 20 for yourself.
Thank you,
Jojo