the answer to this is chemical change
The average mass of the pennies in the sample is 2.16 g.
We have a set of pennies from which 10.00% have an average mass of 2.15 g and 90.00% have an average mass of 2.156 g. The average mass (am) of the whole set is a weighted average, that considers the mass (m) and the percentage (perc) of each group. We can calculate it using the following expression.

The average mass of the pennies in the sample is 2.16 g.
You can learn more about weighted average here: brainly.com/question/18554478
The average atomic mass of iron : 55.85 amu
<h3>Further explanation
</h3>
The elements in nature have several types of isotopes
Isotopes are atoms whose no-atom has the same number of protons while still having a different number of neutrons.
An atomic mass unit = amu is a r<em>elative atomic mass of 1/12 the mass of an atom of carbon-12.
</em>
The 'amu' unit has now been replaced with a unit of 'u' only
Atomic mass is the average atomic mass of all its isotopes
atomic mass X = mass isotope 1 . % + mass isotope 2.%
...
The average atomic mass of iron

It is true, because a strong magnet that can be turned off and on is an electromagnet.