The point at the base of the mountain is described as point zero, that is they are still at sea level. If Melissa now moves to a position 27 metres above sea level, that is positive 5, assuming we are now measuring along a vertical number line. Brian however moves to a location which is 21 metres below sea level, or negative 21 along the vertical number line. Since we are measuring distances, the distance from point zero to both hikers' new position will be measured in absolute values only, that is both distances will be measured as positive.
Hence the distance between Melissa and Brian is;
Distance = 27 + 21
Distance = 48
The difference in altitude between both hikers is 48 metres
