If a credit card has a balance of $120 for 10 days, a balance of $150 for the next 12 days and a balance of $90 for 8 days, what is the average daily balance for that 30 day period?

Average balance=total number of dollar-days / total number of days

Total number of dollar days
= 120*10+150*12+90*8
=1200+1800+720
=3720 dollar-days

Total number of days
= 10 + 12 + 8
=30 days

Average daily balance 
= Dollar-Days / Days
= 3720 dollar days / 30 days
= 124 dollars