Question 65758
YOU ARE RIGHT ON TARGET WITH a and r,  BUT:
Where did you get n=126??

The series breaks into:  2x5^0+2x5^1+2x5^2+2x5^3+2x5^4

Try to run the numbers with n=5

Sn=a((r^n)-1)/(r-1)

S=2((5^5)-1)/(5-1)

S=2(3124)/4

S=3124/2=1562

Hope this helps----ptaylor