Micrometer or vernier caliper
METERS!!!The unit of measurement you would use to measure the width of your classroom is meters.
millimetres
centimeter?
Centimetre.
Parsecs
You would use millimetres.
You can use a penny for that. Put in between the threads then mark and measure the depth on the penny.
A tape measure.
If you don't have enough other information about the triangle to allow you to calculate the width, then you have to use a ruler or a tape measure.
a ruler
inches
Millimetres