He used 1/3 of his tank because 1 - (2/3) = 1/3
He used 4 gallons of gas because 12 x (1/3) = <<12*(1/3)=4>>4
He drove 20 miles because 2 x 10 = <<2*10=20>>20
He gets 5 miles per gallon because 20 / 4 = <<20/4=5>>5
#### 5