????

Call the amount at 60%  =  x

Call the amount at 15%  =  450 - x

So

.60x + .15(450 - x)  =  .25 (450)   simplify

.60x + 67.5  -  .15x =   112.5

.45x  =  45    divide both sides by .45

x = 100 gallons ......this is the amount at 60%

450 - x  =  450 - 100  =  350 gallons  = amount at 15%