Answer:
7.9
Explanation:
When we put the metal piece in the liquid (which is in the graduated cylinder), how much it goes up is equal to the volume of the piece we inserted.
So now we know that the volume of that piece of unknown metal is 7mL (which is the same as 7
).
Density is
.
So the density of that piece of metal is 
Which leaves us with a final density of 7.9
It is a battery (the long and short lines represent 2 cells)
If iron were used to make coins then it would rust easily,because of its exposure to water, air etc..