Let x represent the horse age
Let y represent the human age
Since the horse age and human age vary directly, we have the relation

Introducing an equality sign, we

Where K is a proportionality constant, evaluated as

Since a 5-year-old horse is equivalent to 15-year old human,
x = 5
y = 15
we have

Thus,

Thus, for a horse who has lived 16 years
x = 16