Final answer:
The ratio of 4 airplanes to 420 people simplifies to the fraction 1/105, which is the simplest form.
Step-by-step explanation:
The ratio of 4 airplanes to 420 people is given by the fraction 4/420. To simplify this fraction to its simplest form/lowest terms, you would divide both the numerator (4) and the denominator (420) by their greatest common divisor, which is 4. This simplifies the fraction to 1/105, as 4 ÷ 4 is 1 and 420 ÷ 4 is 105. Therefore, the ratio of airplanes to people in its simplest form is 1/105.