Question

Find the sum of the first 10 terms for the sequence {21, 63, 189, . . . }.

Answer 1

It’ll be 620,004
Each number is multiplied by 3 so the 10 numbers would be
21; 63; 189; 567; 1,701; 5,103; 15,309; 45,927; 137,781; 413,343
Adding those numbers equals 620,004

Answer 2

The sum of n terms of a geometric sequence with first term "a" and common ratio "r" is given by
$S_{n}=a\dfrac{r^{n}-1}{r-1}$
You have a=21, r=3, so the sum of 10 terms is
$S_{10}=21\cdot \dfrac{3^{10}-1}{3-1}=620004$

The appropriate choice is the 2nd one:
   620,004