Refer to figure.
Note: The given compass heading cannot be directly converted to true heading. First, it must be converted to magnetic heading using the given deviation, and then to true heading using the given variation.

VARIATION

Magnetic variation is the angle between the direction of True North and Magnetic North, expressed in degrees East(+) or West(-).

The following mnemonics can be used to convert True Heading to Magnetic Heading:

• Variation East (+) → Magnetic Least, where “Least” means that the Magnetic Heading is less than the True Heading.
• Variation West (-) → Magnetic Best, where “Best” means that the Magnetic Heading is greater than the True Heading.

DEVIATION

Compass deviation is the angle between the direction of Magnetic North and Compass North, expressed in degrees East(+) or West (-).

The following mnemonics can be used to convert Magnetic Heading to Compass Heading:

• Deviation East (+) → Compass Least, where “Least” means that the Compass Heading is less than the Magnetic Heading.
• Deviation West (-) → Compass Best, where “Best” means that the Compass Heading is greater than the Magnetic Heading.

1. Convert the Compass Heading to Magnetic Heading

• Compass Heading = 175°C
• Deviation = 4°W
• Deviation West (-) → Compass Best
• Magnetic Heading is: 175°C - 4°W = 171°M

2. Convert the Magnetic Heading to True Heading

• Magnetic Heading = 171°M
• Variation = 18°W
• Variation West (-) → Magnetic Best 
• True Heading is: 171°M - 18°W = 153°T