SInce they put money in tha account at the beginning of the month....starting with month #2 , this is an 'Annuity due' calculation rather than an 'ordinary annuity'.....
FV = A [ ((1+i)n -1 ) / i ] * (1 + i)
A = $ 100
i = interest per period in decimal form .068/12 (there are 12 periods per year...monthly)
n = number of periods = 18 * 12 - 1 (-1 because they did not deposit the first month)
= 215
A = 100 (1+.068/12)215 / (.068/12) * (1 + .068/12) = $ 59806.49
The interest will be this amount minus the amount they deposited which was 215 * 100