To find the cost of each of the first three tires, we can denote the cost of each of the first three tires as \( x \). The cost of the fourth tire, according to the promotion, is $5.
The total cost for the tires can then be expressed as: \[ 3x + 5 = 237.50 \]
To isolate \( x \), we first subtract $5 from both sides: \[ 3x = 237.50 - 5 \] \[ 3x = 232.50 \]
Next, we divide both sides by 3 to solve for \( x \): \[ x = \frac{232.50}{3} \] \[ x = 77.50 \]
Thus, the cost of each of the first three tires is \( \boxed{77.50} \).
Therefore, the correct response is B) $77.50.