Question

If morgan spende 17.50 on gas

Answer 1

With $17.50 she can get 11 2/3 gallons. 11 (she just got) + 8 (that she had) = 19 gallons. 19 x 27 MPG = 513 Miles. 
27 divided by 3 = 9 (9 is a third of the MPG) 9 x 2 (since it was 2 thirds that were left) = 18
In total she can drive 531 miles.