Answer:
10 gallons of gas
Step-by-step explanation:
Create a proportion, where x is the number of gallons she will use if she drives 75 miles:
= 
Cross multiply and solve for x:
15x = 150
x = 10
So, she will have to use 10 gallons of gas to drive 75 miles
10 + 10 = 20.
Hope this helps! :)