You can put this solution on YOUR website! figure out sum( 2(5^n), n=0, 3 )
n=0: 2*5^0 = 2*1 = 2
n=1: 2*5^1 = 2*5 =10
n=2: 2*5^2 = 2*25=50
n=3: 2*5^3 = 2*125=250
---------------------------
Adding these you get 312
========================================
Cheers,
Stan H.