need help with dax

I have the table below where in i have original currency and transaction currency along with effective date and exchnage rate , 

i have this requirement where in which i want to create a table with with highest date when i have two or more exchange rates within same month and i want to have a column in which i would like to have the previous month exchnage rate if the current exchnage rate is not available.

@mgwena @cham @amitchandak @Greg_Deckler @Mariusz 

Input table :

Output Table :