You save exist1 the first day of the month, exist2 the second day, exist4 the third day, continuing to double your savings each day. What will your savings be for the first 11 days? Your savings will be exist for the first 11 days. Solution Savings = 1 + 2 + 4 + 8 + 16..... This is a geometric series with common ratio (r) = 2 The sum of n terms of a Gp (Sn) = Sn = a1 x (rn - 1)/(r - 1) where Sn= Sum till nth term a1 = 1st term r = Common Ratio Hence, Savings till 11th day = 1 x ( 211 - 1)/(2 - 1) => 2047.