20 gallons of x% + 60 gallons 12% = 80 gallons 10%
80 gallons 10% etahnol = 8 gallons of ethanol
60 gallons 12% ethanol = 7.2 gallons of ethanol
Therefore the 20 gallons of x% contains .8 gallons of ethanol
.8 / 20 * 100 = 4 %
The concentration of the unknown solution is 4%