The initial amount of bacteria is 50.
In 12 minutes, bacteria doubles.
So, the amount of bacteria after first 12 minutes is,
After the next 12 minutes, the amount of bacteria is,
Again, after the next 12 minutes, the amount of bacteria is,
Again, after the next 12 minutes, the amount of bacteria is,
Now, the total time taken is 12+12+12+12=48 minutes.
Therefore, after 48 minutes, the amount of bacteria is 800.