Keeping $20 to be able to buy gasoline money is performing store of value.
A store of value is basically an asset, entity, or currency that can be kept, retrieved, and traded in the future without degenerating in value. One of the roles of money in an economy is that it acts as a store of value. It is something that someones use to share purchasing power from the present to the future. While cash is an asset that can store value, it's not the only kind. For example, Gold and silver act as stores of value.
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