You would use a magnet to determine what property of a mineral?

Magnetic properties are useful for identifying a mineral, for if observed it can pinpoint a mineral. The most effective testing results are obtained with the use of a powerful magnet. The only minerals that possibly respond to magnets without heating are opaque, metallic-looking minerals.

the value of the 4 in the hundreds place is 400 while the value of th e 4 in the tens place is 40

400 is way bigger than 40.