The given equation is: E=17/20 D , where E=amount of Euros and D= amount of U.S dollar. 
 
Now for 1 U.S dollar, we need to plug D=1 into the above equation. So, we will get......... 
 E=17/20(1)
E=1/20=0.85
0.85 Euros have the same amount as 1 U.S dollar. 
 
HOPE IT HELPS (◕‿◕✿) SMILE!!