Question

Your friend sends out a chain letter e-mail to 12 people and by the next day 15 people have received the letter. Assuming an exponential growth pattern, how many new people will receive it by the 30th day?

Answer 1

If by "new" you mean all except the original 12, then
12((15/12)^30 - 1)
Extra credit: how many people receive it on the 30th day?