The cost of 6 gallons of gasoline is $13.38. 

So, 

the cost of 1 gallon of gasoline will be 13.38 divide by 6,

which will be, $2.23 .

now, for 13 gallons, cost will be,

13 into 2.23, which is,

  2.23

x   13 

-------

  22.30

+  6.69

_______

  $28.99