An owner of a company spends $25 on office supplies. the owner is left with $20. write an equation to find how much money he originally had.

$20+$25=$45 therefore the owner of the company originally had $45

Quick answer = [$45.00]
If he spent $25.00 and he is left with $20.00 then... 

  $25.00
+$20.00
= $45.00 

{Example}  Lets say you had $45 and you went to buy some fast food.. okay in the end your whole meal cost $25 so if your taking that $25 out of your $45 then you would be left with $20. So yeah