Question 348076
1 gallon should equal 60%. Let w water gallon added : the total is 1+w gallon, of which 60% is antefreeze, hence 1 gallon : 
 
0.6*(1+w)=1=0.6+0.6w
 
0.4=0.6w
 
w=4/6=2/3~0.66 gallons.
 
Verif : Total : 1+2/3, % antifreeze : 1/(1+2/3)=3/(3+2)=3/5=60/100.