1. To find the weight (in kilograms) of a ream containing 500 sheets of a given size in inches and gram-weight.

[Length (in inches) x width (in inches) x gram-weight] / 3,100

2. To find the weight (in kilograms) of a ream containing 500 sheets of a given size in centimeters and gram-weight. 

[Length (in cm) x width (in cm) x gram-weight] / 20,000

3. To find the gram-weight of a ream containing 500 sheets of a given size in inches and weight (in kilograms).

[Weight (in kg) per ream x 3,100] / [Length (in inches) x width (in inches)]

4. To find the gram-weight of a ream containing 500 sheets of a given size in centimeters and weight (in kilograms).

[Weight (in kg) per ream x 20,000] / [Length (in cm) x width (in cm)]