Which of these prices is lower than 5 rentals for $9 comparing price per rental?

a) 3 rentals for $6
b) 8 rentals for $16
c) 6 rentals for $10
d) 10 rentals for $19

So don't take my word i'm not exactly a mathematical person but i would assume a. because while selling the soap every month you increase in sells so you would by 11 percent. the you would get the factor 0.89 which if i had to guess would be the monthly profit over 11 probably wrong tho