We are given that \( \tan x^\circ = \frac{7}{5} \).
In a right triangle where the angle \( x^\circ \) is one of the angles, the relationship \( \tan x = \frac{\text{opposite}}{\text{adjacent}} \) holds, where:
- the opposite side to angle \( x^\circ \) corresponds to the segment along First Street (10 miles),
- the adjacent side corresponds to the segment along Main Street, which we need to find.
Using the definition of tangent, we can write:
\[ \tan x = \frac{\text{opposite}}{\text{adjacent}} = \frac{10}{d} \]
where \( d \) is the distance car B travels on Main Street.
Setting the two expressions for \( \tan x \) equal gives:
\[ \frac{10}{d} = \frac{7}{5} \]
Now, we cross-multiply to solve for \( d \):
\[ 10 \cdot 5 = 7 \cdot d \] \[ 50 = 7d \]
Divide by 7:
\[ d = \frac{50}{7} \approx 7.14 \]
Finally, rounding to the nearest hundredth:
\[ d \approx 7.14 \text{ miles} \]
Thus, car B will have to travel 7.14 miles on Main Street to reach Oak Street.
So, the answer is 7.14 miles.