How to calculate for every day but different value?

Hi,
I have a table exchange_rate which has currency, date,… In another table expense it has FK from exchange_rate and a column amount.
The currency can be changed once per day and it can have the same value several days in a row but it can be changed every day. I have to get how much in some value of money in currency value (i.g. euros). And to calculate the total value how much that in euros from all days.

amount / currency (every day different) = euro.
I am not sure how to do this when I have