An atom has 18 protons, 18 electrons, and 20 neutrons.
this atom has ?
a negative charge
an atomic number of 20
a mass number of 38
a positive charge

a mass number of 38

Explanation:

X This atom has no a negative charge .

because, number of  protons (18) & electrons (18) are same. so,This atom is neutral.

X This atom has an atomic number of 20.

Because, it has 18 protons. so, This atom has an atomic number of 18.

X  This atom has a positive charge

because, number of  protons (18) & electrons (18) are same.so,This atom is neutral.

d₁ = displacement of tern from arctic to antarctic = 11000 miles

d₂ = displacement of tern from antarctic to arctic =   - 11000 miles

d = total displacement for the journey

total displacement for the journey is given as

d = d₁ + d₂

d = 11000 + (-11000)

d = 0 miles

t₁ = time of travel from arctic to antarctic = 3 months = 3 x 30 days = 90 days

t₂ = time of travel from antarctic to arctic = 3 months = 3 x 30 days = 90 days

t = total time of travel

total time of travel is given as

t = t₁ + t₂

t = 90 + 90

t = 180 days

average velocity is given as

average velocity = total displacement/ total time

v = d/t

v = 0/180

v = 0 miles/day