Answer:
58593
Step-by-step explanation:
The sum to n terms of a geometric sequence is
[tex]S_{n}[/tex] = [tex]\frac{a(r^n-1)}{r-1}[/tex]
where a is the first term and r the common ratio
r = 15 ÷ 3 = 75 ÷ 15 = 5 and a = 3, hence
[tex]S_{7}[/tex] = [tex]\frac{3(5^7-1)}{5-1}[/tex]
= [tex]\frac{3(78125-1)}{4}[/tex] = [tex]\frac{3(78124)}{4}[/tex] = 58593
