The kinetic energy formula is \frac{1}{2}mv^{2}, where m is the mass and v is the velocity in km/hr.

The mass of both the objects are the same, but the velocity for the first car say v_{1}= 90 \frac{km}{hr} = 25 \frac{m}{s}

The velocity of the second car say v_{2} = 54\frac{km}{hr} = 15 \frac{m}{s}

So, we have the ratio of K.E of the two cars as

\frac{1}{2}mv_{1}^{2} : \frac{1}{2}mv_{2}^{2}

The like terms gets cancelled and we are left with v_{1}^{2} : v_{2}^{2}

Plugging in v_{1} = 25 and v_{2}= 15 we get 25^{2} : 15^{2}

= 625 : 225

= 25 : 9

**So, the ratio of the KE of the first car to the KE of the second car is 25 : 9 **

Here is how you can convert km/hr to m/s

https://byjus.com/questions/how-do-you-convert-km-h-to-m-s/

Here is how you can express a ratio in the simplest form