The cost per gallon is $10
Here, we want to get the cost per gallon
From the question, a peck is 1/4 of a bushel
Hence, a bushel is 4 times the price of a peck
So, for a peck of $20; the price of a bushel will be;
[tex]\text{ \$20 }\times4\text{ = \$80}[/tex]Furthermore, a bushel is 8 gallons
So, the cost of a gallon is 1/8 of the price of a bushel
The cost of a gallon is thus;
[tex]\frac{1}{8}\times80\text{ = \$10}[/tex]