Answer:
$ 4096 on the 7th day
Explanation:
From the amount earned for the first 3 days it's 1 , 4 , 16 , .... We realize it increases exponentially.
So therefore:
Day 1 =4⁰
Day 2=4¹
Day 3=4²
Day 4=4³
Day 5=4⁴
Day 6=4⁵
Day 7=4⁶
Therefore on day 7 he saves 4⁶ which is $4096