Question 117423
Call his lifespan {{{s}}}
{{{(1/4)s + (1/5)s + (1/3)s + 13 = s}}}
{{{(15/60)s + (12/60)s + (20/60)s + 13 = s}}}
{{{15s + 12s + 20s + 13*60 = 60s}}}
{{{47s + 780 = 60s}}}
{{{13s = 780}}}
{{{s = 60}}}years old answer