Let initial bacteria, P0=10000.
A colony of 10,000 bacteria doubles in size every 29 hours.
So, in first 29 hours, the bacteria becomes 20000.
In the next 29 hours, the bacteria becomes 40000.

Now, 29+29=58 hours.
In the next 29 hours, the bacteria becomes

Now, the total hours is 58+29=87 hours.
So, in 87 hours the bacteria becomes 80000.