The key calculation in this problem is figuring out how many times 80 goes into 1,000,000. I'll build up in steps here.
80 x 5 = 400. This gives us a building block on our way to 1,000,000. From here, we can go further and say that 400 x 5 = 2,000; that 2,000 x 5 = 10,000; and finally that 10,000 x 100 = 1,000,000. Altogether, starting from 80, that's
80 x 5 x 5 x 5 x 100 = 80 x 25 x 500 = 80 x 12,500
So, since 80 goes into one million 12,500 times, it takes 12,500 minutes for the animal's heart to beat that 1,000,000 times.