Question

X has a collection of rare coins on first day he sold one coin, on second day two coins and on third day 4 and so on for 8 days. How many coins has he sold in 8 days?

• A. 125
• B. 221
• C. 255
• D. 190

Answer 1

The correct option is C 255

solution:

its a geometric progression

1 , 2, 4 , 8, so on (r = 2)

sum of g.p. is

$S_n=a\left[\frac{(r^n-1)}{(r-1)}\right]$

$S_n=2\left[\frac{(2^8-1)}{(2-1)}\right]$

$S_n=2\left[128\right]$

$S_n=254$+1 (first number of series is 1) = 255

hence the correct opt: C