Question 1061781
Second day: 200
first day: 100 
So we have a series with a ratio of 200/100 = 2
Now, the sum of a geometric series S with n terms and with a ratio r is:
S = (first term)(1-r^n)/(1-r)
Since we have a first term 100 and a ratio of 2, to find the sum after n days we do this:
= 100(1-2^n)/(1-2)
-.-.-.-.-.-.-.-.
Let's test this formula. Let's say that he's been saving for 4 days:
100(1-2^4)/(1-2) = 1,500
Compare with this:
Day 1 = 200-100 = 100
Day 2 = 400-100 = 300
Day 3 = 800-100 = 700
Day 4 = 1600-100 = 1500
So our formula works.