Answer:
666.7
Explanation:
We know the bacteria double every 18 hours. The initial population is 50.
After 18 hours - 100
After another 18 hours - 200
After another 18 hours - 400.
We are now up to 54 hours. With just 6 more hours to go, by how much does the bacteria increase? 6 hours is 1/3 of 18 or 0.3333.... If we had another 18 hours the bacteria would have grown to 800. Instead it grows a 1/3 of this.

The bacteria is 400+266.67 = 666.7 at 60 hours.