1. You have that the series is: 5+25+125+625+3125+15625
2. You must find the ratio (r) between the adjacent members. Then, you have:
25/5=5
125/25=5
625/125=5
3125/625=5
15625/3125=5
3. Therefore, the ratio is:
r=5
4. Then, each term has te form 5^k. So, you have:
5^1=5
5^2=25
5^3=125
5^4=625
5^5=3125
5^6=15625
5. As you can see, "k" goes from 1 to 6.
6. The answer is shown in the image attached.