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.
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To solve the concentration, it is equal to the mass of the calcium ions divided by the total mass of the solution.
C = mass of ions / mass solution
C = ( 8.5 x 10 ^-3 g ) / 490 g
C = 1.7 x 10^-5
since the choices are in ppm, multiply it by 10^6
C = (1.7 x 10^-5 ) 10^6
C = 17 ppm
.0071 you round 0 to1 because the 6 is bigger than 5 and if anything is bigger than five you round up that last number
Answer:
Scientists found that seams formed mountain ranges that ran along the middle of some ocean floors. Scientists found that mid ocean ridges extend into all of earth's oceans and that a steep sided valley splits the tops of some mid ocean ridges.
Explanation: