Answer :

The percent decrease we are going to use is "x" 
If something decreases by 30%, then you are actually multiplying the number by 70%, which is 100% - 30% = 70%. 
Therefore: 

60 (1 - x) = 39 
60 - 60x = 39 
-60x = -21
x = 0.35 or 35%

The percent decreased 35% from 60 to 39