pro ra·ta /(ˌ)proˈretə, ˈrɑ, ˈræ-/
  Pro ra·ta  In proportion; proportionately; according to the share, interest, or liability of each.
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  Pro, prep.  A Latin preposition signifying for, before, forth.
  Pro confesso 
  Pro rata. 
  Pro re nata 
  pro rata
       adv : in proportion [syn: proportionately]