Your answer should be “C.”
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Answer:- 12 km = 12000 m
Solution:- It's a metric unit conversion where we are asked to convert 12 km to m where km stands for kilometer and m stands for meter.
In metric conversions, kilo means 1000.
So, 1 km = 1000 m
It means, we multiply the given km by 1000 to get the answer in m as:

= 12000 m
Hence, 12 km = 12000 m.
Box C will have the greatest density.
All boxes have the same volume.
Explanation:
We calculate the density using the following formula:
density = mass / volume
density of Box A = 10 g / 20 cm³ = 0.5 g/cm³
density of Box B = 30 g / 20 cm³ = 1.5 g/cm³
density of Box C = 170 g / 20 cm³ = 8.5 g/cm³
Box C will have the greatest density.
All boxes have the same volume.
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There are two naturally occurring isotopes of gallium: mass of Ga-69 isotope is 68.9256 amu and its percentage abundance is 60.11%, let the mass of other isotope that is Ga-71 be X, the percentage abundance can be calculated as:
%Ga-71=100-60.11=39.89%
Atomic mass of an element is calculated by taking sum of atomic masses of its isotopes multiplied by their percentage abundance.
Thus, in this case:
Atomic mass= m(Ga-69)×%(Ga-69)+X×%(Ga-71)
From the periodic table, atomic mass of Ga is 69.723 amu.
Putting the values,

Thus,

Rearranging,

Therefore, mass of Ga-71 isotope is 70.9246 amu.