Which of the four cars had the least net force applied to it when it was launched down the ramp

The least net force applied : Car 3(12 N)

Newton's 2nd law explains that the acceleration produced by the resultant force on an object is proportional and in line with the resultant force and inversely proportional to the mass of the object  

∑F = m. a  

Car 1 ⇒m=0.5 kg, a=36 m/s²

Car 2⇒m=0.8 kg, a=50 m/s²

Car 3⇒m=0.6, a=20 m/s²

Car 4⇒m=1, a=19~m/s²

40 degrees Celsius

Explanation:  

Equation to use: P1/T1   = P2/T2

First: add 273 K to 20 C to get 293        

Then plug the numbers into the equation: 4.50/293 = 4.78/x    

Then you want to cross multiply: 4.50x = 1400.54

Then solve for x:  x = 313 K

Then convert back to celsius: 313 – 273 = 40 C