78 months is 78/12 years=6 years and 6 months. There are 52 weeks in a year (52 weeks and one day=365 days) and 26 fortnights. So in one year at $85 per fortnight we have 26*85=$2,210. In 6 years this comes to $13,260. 6 months comes to $1,105 (13 fortnights). 6 years contains at least one and no more than 2 leap years, so the extra days accumulated over 6 years is 7 or 8 days, less than a fortnight. Therefore in 78 months 13,260+1,105=$14,365 has accumulated.
Another apparent way to work this is out is to say that there are 2 fortnights in a month, so 78 months is 156 fortnights, but 156*85=$13,260. The discrepancy arises because most calendar months have more than 28 days while a fortnight always has 14 days, so it's important to account for these extra days. That's why I worked from 26 fortnights (rather than 24) a year. 6.5*26*85=$14,365.