By keenly investigating the given above, it is observed that the given is a geometric sequence with first term (a1) equals 1 and common ratio (r) is 1.5. The geometric series can be calculated by the equation,
Sn = a1 x (1 - r^n) / (1 - r)
where n is 15.
Substituting the known values,
Sn = 1 x (1 - 1.5^15) / (1 - 1.5) = 873.78778
Therefore, S15 is approximately equal to 873.79.