How fast should a car move for the driver to perceiver a red traffic light `(lambda ~~0.70 mum)` as a green one `(lambda' ~~ 0.55 mu m)`?

Explanation

As in the previous problem
`(v)/(c ) = (lambda^(2)-lambda'^(2))/(lambda^(2)+lambda'^(2))`
so `v=c(((lambda)/(lambda'))^(2)-1)/(((lambda)/(lambda'))^(2)+1) = 7.1 xx 10^(4)km//s`