Every geometric series is defined by a starting point and a common ratio. You start from the starting point, and you keep generating new items by repeatedly multiplying by the common ratio.
In this case, the starting point is 10, and the common ratio is 5, because every next term is given by multiplying the previous one by 5:
- Start with 10
- Multiply it by 5 to get 50
- Multiply it by 5 to get 250
- Multiply it by 5 to get 1250
- Multiply it by 5 to get 6250