Refer to figure.
The pressure altitude of 9000 ft is not provided in the table; instead, values are given for 8000 ft and 10 000 ft. Therefore, the endurance must first be determined at both 8000 ft and 10 000 ft, and then interpolated by taking the average of these values to determine the endurance at 9000 ft.

• From the pressure altitude of 8000 ft, move across the table to the corresponding RPM of 2400, and then read the endurance of 6.8 hr.
• From the pressure altitude of 10 000 ft, move across the table to the corresponding RPM of 2400, and then read the endurance of 7.6 hr.

Endurance at 9000 ft

• Endurance at 9000 ft = (Endurance at 8000 ft + Endurance at 10 000 ft) / 2
• Endurance at 9000 ft = 6.8 hr + 7.6 hr / 2
• Endurance at 9000 ft = 7.2 hr