Answer: 17 years
Explanation:
Expression for rate law for first order kinetics for radioactive substance is given by:

where,
k = rate constant
t = age of sample
a = let initial amount of the reactant
a - x = amount left after decay process
a) for completion of half life:
Half life is the amount of time taken by a radioactive material to decay to half of its original value.


b) for 8900 g of the mass of the sample to reach 7700 grams


Thus it will take 17 years
A nonpolar molecule is entirely symmetric. For a molecule to be nonpolar, it must have at least a linear, trigonal planar, or a tetrahedra shape. However, that’s not totally enough. For the molecule to be completely symmetric, aside from having a symmetric shape, all of the atoms that are connected to the central atom must be alike. Therefore, symmetry has two components, they are the geometric arrangement of the outer atoms and whether or not they are all the same.
Fundamentally, non-uniform or it is the uniform, distribution of electrons that ascertains if a molecule is polar or nonpolar, but this distribution is established by the dispersal and distinctiveness of the atoms in a molecule.
Here is some different pics of a water table. I hope this is what you are looking for :). You can click the pictures to in large them.