Answer: Every 60th caller would win both.
Step-by-step explanation:
Since we have given that
every 12th caller receives a free concert ticket.
And
every 15th caller receives a limo ride.
We need to find the first caller who would win.
So, for this we would find L.C.M. of 12 and 15.
And we know the least common multiple of 12 and 15 is 60.
Hence , every 60 th caller would win both.