What is dimensional analysis? a. A systematic approach to problem solving that uses conversion factors to move from
one unit to another.
b. A ratio of equivalent values used to express the same quantity in different units; is
always equal to 1 and changes the units of a quantity without changing its value.

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explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.  

that's what i got you can count the number of atoms