This can be solved by ratio and proportion. The first condition is
3 1/2 hours is to 1/3 tank of gas
The proportion relation is
3 1/2 : 1/3 :: x : 1/2
where x is the number of hours that Tanya can drive
7/2 / (1/3) = x / (1/2)
Solving for x
7/6 = x/2
x = 14/6 or 2 and 1/3 hours