Hey there :)
An exact answer cannot be given as it requires to reseach about banks and co-operatives in your locality. However, I can give you a hint.
Research banks or cooperatives in your location that has the greatest percentage in terms of investment or savings.
Let's say, for example, Bank ABC has 5% annual simple interest in their Savings Account.
If you invest your entire money 100,000 pesos at Bank ABC with 5% annual interest for 20 years, you will earn an interest of:
Hence, after investing at Bank ABC for 20 years, you will earn 100,000 pesos on top of your principal/invested amount that is 100,000 pesos. Your money will become 200,000 pesos now.
So, with this, find a bank/cooperative that has the highest return rate in your location. Once you have decided bank/cooperative, research further with regards to the background of the company, other services offered, application requirements, etc.
~ Benjemin360