.
All we need to do is to substitute the given frequencies into equation
239 = .
To solve it, cross multiply, simplify and find v
239(761 + v) = 256*761
761 + v = = 815.1 mph
v = 815.1 - 761 = 54.1 mph.
ANSWER. The speed of the car is 54.1 miles per hour (rounded as requested).
Solved.
------------------
This problem (its TWIN version) was just solved once at this forum under the link
https://www.algebra.com/algebra/homework/equations/Equations.faq.question.1181384.html
https://www.algebra.com/algebra/homework/equations/Equations.faq.question.1181384.html