What must quantities have before they can be added or subtracted?

Same units

Explanation:

The quantities can be added or subtracted if they have same base units. For example, adding length to mass is meaningless but length can be added or subtracted to length.

Small units can be converted to standard units like in case of length, if one measurement is in centimeters and other in meters and you want add or subtract, then convert cm to m first.

1 cm = 10⁻²m

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