Question

A helicopter is flying at 40 feet above the sea level and a submarine is at 60 feet below the sea level. On the number line, the helicopter is at ft from the sea level and submarine is at ft from the sea level.

Answer 1

Above the sea level, distances are represented by postive numbers and below the sea level, distances are represented by negative numbers.

Helicopter is flying 40ft above the sea level, so it is at +40 ft from the sea level on the vertical number line.
Submarine is 60 ft below the sea level, so it is at $-60\text{ft}$ from the sea level on the vertical number line.