Question

12. Sn = 1550, n = 3, r = 5
13. Sn = 1512, n = 6, r = 2
14. Sn = 6854.4, r = 2, an = 3481.6
15. Sn = 4860, r = 3, an = 3280.5

Answer 1

Need to solve for a1

Answer 2

so plug in the formula
Sn = a1 (r^n - 1)/(r-1)
or, if you have an, and need n first, let's do #14
an = a1*2^(n-1) = 3481.6
Sn = a1*(2^n - 1)/(2-1) = a1*2^n - a1
6854.4 = 2an - a1 = 6963.2 - a1
a1 = 108.8