Solution:
Introductory APR for six months = 0%
Minimum payment per month = $ 20
When , the introductory period gets over, the APR= 15 %(Yearly)
Monthly APR

Total balance, in Laurie's account when the introductory period gets over= $ 135
So, Minimum payment,when the introductory period ends
= 135-20×6
= 135 - 120
= $ 15
=Option (A)
But if you consider APR of 15 % for 7nth month,
Then total Payment , if rate of APR is for 12 months or for the period of 6 months
$