The fraction of cars in the lot that are either white or blue is the sum of the fraction of cars that are white and the fraction of cars that are blue, minus the fraction of cars that are both white and blue (since we don't want to count these cars twice).
The fraction of cars that are white is 4/12, and the fraction of cars that are blue is 3/12. To find the fraction of cars that are both white and blue, we need to find the greatest common factor (GCF) of 4 and 3, which is 1. Then, we can divide both 4 and 3 by the GCF to get the simplified fractions 4/12 = 1/3 and 3/12 = 1/4. The fraction of cars that are both white and blue is the product of these two fractions, which is 1/3 x 1/4 = 1/12.
Now we can add the fractions for white and blue, and subtract the fraction for white and blue together:
Fraction of cars that are white or blue = 4/12 + 3/12 - 1/12
Fraction of cars that are white or blue = (4 + 3 - 1) / 12
Fraction of cars that are white or blue = 6/12
Simplifying the fraction, we get 6/12 = 1/2.
Therefore, 1/2 of the cars in the lot are either white or blue.