Ojo lived 1/4 of his life as a boy, 1/5 as a young man and 1/3 as a trader and 13 years as an old man. How old is he?

Answer 1

Let L be the length of Ojo's life in years. Then

L = L/4 + L/5 + L/3 + 13

L = L * ({15/60) + {12/60} + {20/60}} + 13

L = L * 47/60 + 13

L * (1 - 47/60) = 13

So

L = 13/(1 - 47/60} = 13 / {13/60) = 60 years