Yes, these are opposite quantities. The first town is 20 feet above sea level, which means it has a positive value of 20. The second town is 10 feet below sea level, so it has a negative value of -10. When you add these two quantities together, 20 + (-10) = 0, the sum is zero.
Yes, the heights of the two towns can be considered as opposite quantities. The first town is 20 feet above sea level, which can be represented as +20, and the second town is 10 feet below sea level, which can be represented as -10. The sum of these two heights, (+20) + (-10), equals 10 feet above sea level. Therefore, the sum of the heights of the two towns is not zero, but rather 10 feet above sea level.
