Using the metric system which is larger 40 cm or 400 mm

There are 100 centimeters in 1 meter, and 1000 millimeters in 1 meter. That means that 100 cm=1000 mm, so 1 cm = 10 mm. Multiply that by 40, and 40 cm = 400 mm.

Technically, I can argue that 40 cm and 40.0 cm are different numbers and it's possible that 40 cm is a larger number than 400 mm based on the precision of the measurement.

Depending on where you are in your education 40.0 cm and 40 cm are two numbers with different precisions.

For now, though, 400 mm and 40 cm are probably the same.

400 mm says the measurement is known to 1 mm and 40 cm says the measurement is known to within 1 cm.

Later, you may learn that the rules for adding change: e.g. 40 cm + 40 cm = 80 cm and 40.0 cm + 40 cm = 80 cm and 40.0 cm + 40.0 cm = 80.0 cm.