This is a 40-meter third-grade math problem. Divide 500 decimeters into 50 meters, and 90-50 meters equals 40 meters. Meter, decimeter, centimeter and millimeter unit conversion:

1 m = 10 decimeter =100 cm =1000 mm.

1 decimeter =10cm =100mm = 0. 1 meter

1 cm =10mm = 0. 1 decimeter = 0.0 1 meter.

1 mm = 0. 1 cm = 0.0 1 decimeter = 0.00 1 meter.

Meter, decimeter, centimeter and millimeter are all units of length, and the international unit is "meter" (symbol "m"). Commonly used units are millimeter (mm), centimeter (cm), decimeter (dm), kilometer (km), meter (m), micron (micron), nanometer (nm) and so on. Length units play an important role in various fields.