The first term of the series?

The sum of a geometric series is equal to $a\frac {1-r^n}{1-r}$ with a the first term of the series and r <1 (in order for the series to converge). Since been tends to infinity we can conclude that $1-r^n$ tends to 1. So we have the equation: $16 = a\frac {1}{1-r}$ . From which follows $a = 16 (1-r)$ . Since we know the first 4 terms sum to 15 we also have $15= a\frac {1-r^4}{1-r} = 16 (1-r^4 )$ . The last equation is easily solved for r: $r=\frac {1}{2}$ . This gives a = 8.