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

answer(20) psi;