The sum of the first 6 terms of a geometric series is 15,624 and the common ratio is 5.
What is the first term of the series?

  4

Step-by-step explanation:

The sum of n terms of a geometric series with first term a1 and common ratio r is given by ...

  Sn = a1·(r^n -1)/(r -1)

for r=5 and n=6, this becomes ...

  15624 = a1·(5^6 -1)/(5 -1) = a1·(15624/4)

Then we have ...

  1 = a1/4 . . . . . divide by 15624

  4 = a1 . . . . . . multiply by 4

The first term of the series is 4.