Final answer:
City Y's population is twenty times larger than City X's, with City X at 300,000 (3 x 10⁵) and City Y at 6,000,000 (6 x 10⁶).
Step-by-step explanation:
City X has a population of 3 × 10⁵ (300,000) and City Y has a population of 6 × 10⁶ (6,000,000). The relationship between the populations of City X and City Y is that City Y's population is twenty times larger than that of City X. To understand this relationship, consider the powers of ten involved: to compare 3×10⁵ to 6×10⁶, align the powers by multiplying the coefficient of City X by 10, giving 30×10⁵, and then compare it to the 6×10⁶ of City Y, showing that the population of City Y is indeed twenty times greater than City X's population because 6×10⁶ is equivalent to 60×10⁵.